My clematis vine is only flowering on the top, should I cut it back?
Make sure the feet are in the shade and the head is in the sun. Check moisture levels of soil.
Make sure they get a general feed in early spring.
Following the pruning information in the link below, you can clean up the vine at the appropriate time.
On a personal note, I had a summer blooming Clematis that was very sparse. I cut it to the ground and 2 years later a beautiful flush of flowers. A very hard prune may be an option for you.