Sounds like you have type III clematis which bloom on new wood and your not pruning them back resulted in fewer blooms this year. Another thing to consider is the type of fertilizer you have used. Was it the same both years? High nitrogen fertilizers will produce tons of foliage and few flowers. Another thing to consider is whether the weather conditions this year were similar to last years. If it was moderately wet the previous year and dry this season and you didn't supplement the plants with additional water, they might have been under stress and this affected their blooming ability. Are they growing near other plants that have gotten larger this past year and competed with them for the available moisture and nutrients.

I would hard prune them back late this fall or early winter after they have gone dormant, make sure they get adequate fertilizer (low nitrogen) and water, and see how they do next year.