I have not come across a non-flowering Wisteria but I have dealt with poor flowering, overgrown and badly pruned Wisteria.
When you say pruned hard can you tell me how exactly you pruned it?
If pruned wrongly they will not have any bud spurs to flower from.
Check this if you are in any doubt about how to prune....
The other reason for no blooms is the state of the soil. Wisteria like a rich, soil that does not dry out too much. If too dry or poor they will not produce flowers.
Hope that hlps. 20 years is a long time to wait for flowers!