Could you put on a photo?, it would help to know where it is growing.

It is growing on a pergola which as you can see from photo has reached the top and has started to go across the top of the pergola. It is in the sun from approx 10am to sunset.
There is plenty of new growth on all six vines. We feed it with potash solution

Sorry the photo didn't seem to come through, but I am able to get an idea from what you have said, the 6 canes you have should be kept apart and trained up your pergola, they will age and form rods. Further canes will develop across the top of your pergola, then you will be able to stop side growth, flowers will appear to form your grapes which you thin out in July. A spring feed of Blood, Fish and Bone should be applied round the root area. Liquid feed with Tomorite May to August once a month. Keep well watered and damp down around your vine on hot days. Hope this will help you.

My answer to any plant that does not do what is expected from it, is - pruning. Usually the new growth should encourage flowering. Too many leaves mean not enough nourishment for anything else because the foliage takes up all that there is. In vineyards you only see a stump and not long trailing vines.