I have always had good luck with a fertilizer called Super Bloom. Here in Texas, I get it at Lowes. I love it, and wouldn't use anything else. I have tried other brands, but will only use this. Another thing I like is something called Super Thrive. It is plant hormones, and I think I have bought it at WalMart and also Lowes. You can mix the two together and water. I don't use anything else.
Well, that's my method-hope this helps! Good luck!

Super Thrive's listed active ingredients are .09% B-1 (thiamine) and .048% 1-napthyl acetic acid (NAA), a synthetic auxin or growth regulator. No other ingredients are divulged and it has no NPK analysis. At recommended dilution rates, you are essentially applying lttle more than pure water to your plants.

To determine how to make your geraniums perform better, it's important to know what kind of potting soil you are using and what you are fertilizing with. A liquid fert that has a higher middle number (like the Super Bloom or MG Bloom Booster) should encourage denser, more compact plants with more flowering stems.

Here is a link that might be useful: Super Thrive or Super JIVE??

I use the Super Bloom 2 times per month, and the SuperThrive maybe 3 times the whole growing season, a little stronger than the bottle calls for. I know there are people who do not recommend it, but I like it occasionally. I use Miracle Grow potting soil.
I recently moved to another state, but in my previous home, I grew red and hot pink geraniums together in flower boxes on the front of my home, and would have people come to my door to ask me what I used.

I don't think there is a problem with fertilizer or watering that you are giving them. What I do see a problem with is the amount of sun they get. Geraniums prefer more sun than 6 hours of sun. While the get enough sun to survive and grow, there is not enough to thrive and have multi blooms.

Mine are in full sun all day--around 15 hours--and the only fertilizer they get is smartcote--a 4 month fertilizer.

I do keep my geraniums from year to year and the older ones give many more blooms than the new ones because they are bigger.