I agree with Alina. Clematis are very hungry plants and most of mine have suffered this year from an excess of rain and lack of warmth after already being challenged by drought all last year. Some of my 40+ clems have done very well, others have struggled then finally put on a show and some have just struggled despite TLC.
I plan on giving them all bonemeal to feed their roots this winter plus a mulch of compost and then big feeds of blood, fish and bone in early spring and clematis food as growth starts in earnest. If they look at all dodgy, I'll give them instant tonic with liquid rose or tomato food once the worst frosts are past in May.