There are many factors which in combination may be causing the vines to grow slowly...just skip down to the last paragraphs to get the short answer...

Be sure that the plants are receiving adequate water...

There is a vast amount of misinformation on the web (and even on seed packets) regarding Ipomoea morning glories and fertilizers...

The term fertilizer is a fuzzy term that is used to refer to a large number of different plant nutrients and this fuzziness causes problems...

The seed companies will often say that the Ipomoea actually prefer poor soil,but that is a misleading statement...

There are 3 completely different macro-nutrients known as the NPK ratio corresponding to N = Nitrogen, P = Phosphorus and K = Potassium...additionally there are about 15 other elements including trace minerals that are considered essential to the growth and health of most plant species...

One of the macro-nutrients i.e., Nitrogen (!) in excessive amounts will often delay flowering or result in few flowers...the plants very much like (!) the nitrogen,but it is the people who want alot of early flowers who do not 'like' the effect that too much Nitrogen has on the flowering...and it is very important to distinguish between how the plants respond to different nutrients and as to whether people prefer the particular response...

The seed companies will sometimes state that the plants should not be fertilized and / or that the plants prefer poor soil...they state it that way because many people think they know what 'fertilizer' is,but if the seed packets started to go into the different types of 'fertilizers' and different nutrients like Nitrogen,Phosphorus,Potassium and other elemental nutrients this may create confusion in the minds of those persons who are not familiar with these terms and the particulars of how the NPK ratio affects Ipomoea morning glory plant response...

Generally,if Nitrogen in excessive (!) amounts is available to the plants,the plants will often respond by producing few and delayed blooms,conversely too little (!) Nitrogen can stunt growth...

Phosphorus is generally bloom stimulating and is a component of many bloom boosters...
I have reviewed many threads / disscussions where people go into alot of reasons why phosphorus bloom boosters are just a 'sales rap'...
I am very familiar with the fact that most Ipomoea are short day plants and flowering is triggered by the gradual shortening of the daylight hours,but the light hours are not an absolute determining factor as the light initiated flowering response can vary with the species,cultivars and even individual plants...short day response is not absolute and many plants can flower 'as if' long day plants and / or displays exhibiting mixed responses are commonly experienced especially by those people growing the plants indoors and as supplemented with some type of artificial lighting...additionally,the flowering response can be modulated by various metabolic states which are greatly influenced by the ratios of nutrients available to the plants...

I can state very definitively that Ipomoea generally respond very well and quickly to phosphorus based bloom boosters...the equivalent of side-by-side comparisons has been done by people who are devoted to Ipomoea and the plants generally flower earlier,more profusely and for a longer period of time than those plants that did not receive a phosphorus based bloom booster...the addition of the phosphorus is apparently doing something in the Morning Glories to initiate and boost flowering response other than simply correcting a phosphorus deficiency...

Trace mineral deficiencies may be present and Morning Glories devotees have found that liquid fish emulsion or liquid kelp may solve and unknown trace mineral deficiency and won't hurt...

I am dedicated to dispelling the myths associated with the fuzzy term fertilizer and how proper nutrition realistically applies to Ipomoea and other genera of Morning Glories...the Myths associated with Morning Glories and the application of different plant nutrients are encountered very often and require consistent and proper clarification...

Generally,Morning Glories are not fussy about the soil requirements and usually grow very well and automatically flower up a storm,but,if the plants are not growing well and flowering very nicely (to the satisfaction of the grower),then the grower may need to take a closer look at what is happening and experiment with making appropriate environmental adjustments...

Try watering the plants more often(especially after sunset as the plants will have more time to absorb the water before the sun evaporates it !) and try a phosphorus based bloom booster...this may solve the problem and it won't impair impair the vegetative growth or the flowering...

The attached link is to other threads where I have addressed the Myths associated with Morning Glories,the fuzzy term fertilizer and proper nutrition...

Hope that helps...

Ron

Here is a link that might be useful: Posts related to Morning Glories and fertilizers

Ron, your response about morning glories is really informative. I started planting them a couple of years ago and just love them. For me, some bloom profusely and others very lightly. I have always been afraid to fertilize them. When you mention, fertilizer with phosporus such as a bloom booster, would Miracle Gro be ok? Thanks.

Hi mindysuewho - Be sure to get a fertilizer that specifically states that it is a bloom booster which will have a relatively low amount of Nitrogen and Potassium relative to the Phosphorus...

Miracle Grow Bloom Booster shows 10-52-10
Other good NPK ratios would be something like 10-30-20

Grotek Monster Bloom Booster NPK 0-50-30
http://www.hydroasis.com/hy/productdetail.aspx?id=259&product=monster-bloom-130-grams

Jack's Classic Blossom Booster 10-30-20
http://www.jacksclassic.com/blossom_booster.html

Greensand generally offers the largest range of trace minerals
http://www.basic-info-4-organic-fertilizers.com/greensand.html
http://www.state.ar.us/agc/greensan1.htm

Maxicrop Seaweed
http://www.maxicrop.com/pages/products_1_seaweed.html

rhizosphere
http://forums.gardenweb.com/forums/load/vines/msg040710098176.html

Remember to give the growing medium the Earthworm test...if the red worms will live in it >the medium is healthy...and if the worms reject the growing medium >then something is wrong and it is unhealthy...

The microflora around the roots transform the nutrients into forms that are more usable to the plants...make sure that your growing medium is alive(!) with healthy micro-flora...

Magnesium and sulfur are nutritional elements related to the formation of healthy reproductive structures...

The extent to which various elements remain bound and /or leach out of any growing medium and the eventual buildup of various salts to toxic levels is an additional factor to consider...

some other related nutritional substances

Hope that helps...

TTY,...

Ron

Woo! I had to bump this thread because I have been searching around the internet forever to find answers on how to get my morning glories to bloom more profusely. I have been fertilizing with a high-phosphorus fertilizer since their first leaves showed up(4-54-4)because high phosphorus is also great for new root growth, but I was afraid that it might have been a mistake to give it to MGs. However the seeds I planted not more than 3-4 weeks ago are already 3 feet tall and twirling all over the place, looking healthy as can be! I feared that maybe the nitrogen levels were still too high and that I should stop, but it seems the plant is just, well, healthy! I will update when the plant blooms.

Thanks for all the advice, Ron!