Q.Clematis
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.
https://www.gardeningknowhow.com/ornamental/vines/clematis/growing-clematis.htm
https://www.gardeningknowhow.com/ornamental/vines/clematis/pruning-clematis-vines.htm
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.