I have always read that to keep wisteria blooming it needs to be pruned hard
each year.

I have 3 Wisteria's and I only prune one of them very hard because of it's
location. It's growing on a trellis next to some stairs leading from the upper
deck to the lower deck. It never has more than 2 blooms, but the other 2 that
just get a little pruning in the fall are always covered with blooms.
Sue in Mi. (zone 5)