Clematis - lack of flowers.

Okay where do you live, what soil are you growing them in, are their roots nicely in the shade and their foliage in the sun? Different Clematis need to be pruned at different times of the year so you really need to know what you have got take a look at http://www.flower-gardening-made-easy.com/clematis.html and see if it helps you

Moon grower is spot on again. It's all to do with old wood and new wood. If your clematii grow on new wood, and you are pruning it all off, it's no surprise you are not getting any flowers.
Of course, if they need old wood, and you have pruned off all the new wood, then you are going to have to wait probably two years before the new wood of next year becomes the old wood of the following year.
Did that make sense?
Additionally, depending on locale, they may well be drying out. Clematii hate that.
Don't forget, most plants will do their utmost to flower, because it is their way of "living forever". No flowers at all, usually means a complete depletion of something at root level, usually water, or some nasty little bug is attacking the flowers buds.
Highly likely to be the former.
Do as Moon growers suggests, and you won't go far wrong.

I have Duchess of Albany. The only clematis I have ever had in this garden. Luckily the label is fixed to the plant and there are loads of buds and I am eagerly awaiting the flowering. With fingers crossed that the The Duchess doesn,t get the vapours with wilt. The Duchess looks like she blooms on new wood (as per label). I have been watering her diligently, as the very wise info given in GOV suggests.

2 other questions - are they in the ground or planted in containers? and what are you feeding them with (if you are feeding them at all)

The duchess is in the ground under the front of camellias facing north.I put a bucket of Miraclgro in rainwater on them every week and water daily as it is very dry near the camellias.The duchess has scrambled all over the clematis and into the laburnum. My fingers are still crossed.