"Neil, I have a wisteria, ...."
I can almost guarantee the next few words: "...but it just won't flower."
That question alone makes up probably half of the questions I get on all vines combined! You are not alone!
Finding the actual reason that a wisteria doesn't flower, however, isn't a precise science. Here are things I try to get people to consider.
(1) Wisterias grow and bloom best in full sun. Plants that get very much shade will not bloom as heavily, perhaps not at all.
(2) You must not prune wisterias in the winter. They set their flower buds in the fall and winter, so pruning, when needed, must be done the day after they finish flowering.
(3) High-nitrogen fertilizers encourage vegetative growth at the expense of flowers. Stay away from your wisterias, as much as possible, with your fall turf feedings.
(4) Young wisterias may not bloom until they become somewhat established, generally after a couple of years.
If all else fails, I suggest that people try shocking their plants by root-pruning them in early September. Cut a slit into the ground 15 inches out from the trunk (30-inch diameter) to cut lateral roots. You're not removing any soil -- just cutting roots in the top 8 to 12 inches of soil. Sometimes that shock helps -- a "survival-of-the-species" kind of thing. That needs to be done in September or by early October, however.