Clematis are a bit complex.they are treated differently according to name. Some are cut back and some are not. Some get out of control, like a montana and need to be taken back, but they usually bloom well anyway. Without a name or picture ...close up, not easy to say anything more. Clematis like feeding....have you done this? Welcome to GoY. someone may have a better answer than me.

Dorjac's right, need to know which variety it is, the requirements are different for all of them. Photograph might help to ID it if you don't know.