I don't think that Wisteria thrives in a pot - it's a very large plant and it may well be suffering and pot-bound.

Thanks for your help
BP

They can be grown in very large containers, but the pruning is crucial - you have to be absolutely rigorous and do it at the right times (February and August, and possibly a little inbetween times) and they usually are grown in this manner as a sort of short standard, with one main stem, and the laterals left to cascade downwards or around. Feeding is essential too in a pot, and the compost should be loam based, so John Innes No. 2 or 3 with added leaf mould or similar.