WHEN you prune has a lot of bearing on if it blooms. If one if going to prune, (and believe me, they always need it!) it should be done right after the new growth of leaves and such gets set. If you prune later in the season or year, you prune off the area the new blooms would have come from.
I trim more than 50% off of mine, but I do it in the Springtime. It will bloom beautifully, before even setting leaves, each year, early Spring. I still have to trim a few runners, vines back, just to try to keep it contained, even after "butchering it" earlier in the year. Once mine is finished blooming, I get busy chopping.
Maybe you are trimming too often, and during the wrong time for it?
A couple of other things, If your plant was started from seed, instead of from cuttings, it can take DECADES to bloom. And Wisteria do NOT like fertilizer. If they get more fertilizer than they like, they will not bloom. They also prefer well drained soil.