Good question - now I have to ask you some questions in order to give you an answer. What variety of Clematis is it? Have you cut it back at all, and if so, when?

Yes, without giving as much information as you can with your question it's impossible to answer it.

It might also be useful to know what position you've got it planted in, the gardens orientation and the soil type :-/

Hi Sednil, and welcome to GOY :)
Have you fed your clematis? They are hungry plants...I give mine granular rose feed in early spring...just fork it in round the base of the plant, and keep them well watered. They also like a nice cool root run, so you could put stones or gravel chippings on the soil around them.
Also, you may have pruned them at the wrong time of year, and that will lead to non-flowering.