Are you pruning them in early spring?
Do you know what their names are?
Are you feeding them high nitrogen fertilizer?
Are they in sun or shade?

BorS asks good questions. Without more info, it is difficult to give suggestions as to why your clematis are not flowering.

Are you fertilising them?
get some cowmanure & bet you see a massive difference in just a few weeks time..
The are HEAVY feeders

There is much controversy over how heavy of feeders clematis actually are in the literature that is out there. In fact, last year I conducted an experiment where I did not supply any supplement fertilizer at all and my clematis were not affected at all. I would be more worried about applying too much quick release fertilizer which is going to produce a lot of green vegetative growth and little or no flowers.

hallo,
very interesting, is your soil very rich?
k

Ii am going to get some manure and try that. It is so annoying waiting this long. I gave them bone meal and no high nitrogen fertilizer. I have hear various opinions about fertilizing them.

clematis fall into different pruning groups. If yours look healthy but just don't have flowers, you may be pruning at the wrong time and cutting off all the older branches that should be bearing the flowers.

High nitrogen fertilizer does NOT encourage flowers!
Manure is high nitrogen.

Are you pruning them in early spring?
Some bloom on new growth and some on old. Improper pruning can delay flowering.

Do you know what their names are?
Would allow us to tell you which pruning group and if they are known to be shy bloomers or shy bloomers when new. All Clematis take at least three years to mature.

Are they in sun or shade?
Some are more shade tolerant than others

Manure is high nitrogen and will burn your plants to death if it is fresh. I am hoping the poster meant composted manure since that has a very low nitrogen content and is slow release.

I get the flowers every year on my clematis and I rarely prune they just come back so nice on the stems. I will prune this year. My question is they say in the sale catalogs that clematis bloom june to sept. but so far all 3 different variety only bloom once in june for about 3 weeks. I have heard that they can bloom twice in one season. Is there a way for me to get them to bloom twice or longer during the summer?

Prune them and fertilize after the first bloom.

I agree with what BorS says. This should work provided you have a long enough growing season between when you prune and when the fall arrives in your area. Also,in my experience, some clematis never rebloom regardless if whether you cut them back or not. I have yet to get rebloom on Purpurea Plena Elegans even with pruning back. Also, don't believe what you read in sales catalogs. They will say anything to sell plants.

It is too late. I have already put Farfard's bagged cow manure on the clematis. I didn't touch the leaves. If they are heavy feeders, what brand of fertilizer is the best for them?

A rose or tomato fertilizer that has more phosphorus than nitrogen encourages blooms over green growth.

Manure is great, it won't harm them but it won't encourage blooms over foliage.

Based on my personal experience, I don't believe any one type is any better than the others as long as you don't dump tons of high nitrogen quick release fertilizer on them. Last year I didn't fertilize at all with anything except for using grass clippings and I saw no difference in blooms. The best bet would be to get a soil analysis done to see what your soil is lacking and what it is not. The local Ag agencies in my area do the testing for free.