If it's stressed I would water it first. It was a very dry spring and it may simply be that ground water levels are low. Give it a 10l bucket of water a day, poured slowly so it soaks in rather than runs off. Repeat daily for a week and then every other day. You could add some liquid seaweed or tomato feed after the first 2 or 3 days.
Vendée - 20kms from Atlantic coast. "We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw
I have Wisteria Alba. Its not a very old plant, this is its third year, and last year it put very little growth on and like you I thought I had done something wrong. So just kept watering, feeding tomato food every so often and over winter mulched with manure. This year it has lots of new whippy growth to cut back, so don't give up on it.
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"We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw