Posted: 07/06/2015 at 14:40
Flowering now indicates a group 2 for pruning which means a light pruning after the first flush of flowers finishes. If you feed it as well, you shoud get a second flush fo flowers later in summer.
However, having said that, I treat all mine as group 3s as their tops tend to get frozen to death in my winters. Every March I prune them back to about 9 inches and give them a generous feed of clematis food followed by several liquid feeds of rose or tomato fertiliser until flowering commences.
Nelly Moser is always first for me and is already in flower with all it stems producing fresh foliage and flowers low down. The others follow in due course. This pruning method also means they don't get too big and there are no bare woody stems.
You could try feeding yours now with both the slow release clematis fertilser and the liquid tomato food to see if it produces new shoots to flower next year. Even if it doesn't it will help grow stronger roots and then you could also try the hard prune and big feed next spring and completely renew your plant. As the new shoots form, train them along horizontal wires attached to your fence at 12 to 18 inch intervals as this encourages extra flowering.