This is going to sound ridiculous, but it does work. A friend asked an old gardener / nursery man friend the same question as her wisteria had never bloomed and it was about 6 years old. She had tried everything the garden shops recommended. He told her to get a rolled up newspaper and go out and smack the wisteria with it all over or at least what she could reach. But don't try to beat it to death. He said they occasionally need to be "jump started". It worked for her.