many of them flower on this years new growth so the flowers are on the tips.
Pruning the clematis down low in the spring will encourage new growth and the flowers will be at the ends of the stems but the whole plant wont have 'bare legs'.
if you know the variety of your clematis check which pruning group it is in and then prune according to the RHS pruning advice.
https://www.rhs.org.uk/advice/profile?pid=109

Also make sure that the sides of the plant get some sun. Any grown on a north-facing wall are likely to only bloom at the top, no matter what you do.