I have a wisteria and it still looks pretty decent, but it's not in the fullest of sun. I have NOT been diligent about watering. My first thought would be lack of water just because our weather has been so blistering hot! In doing some checking, I read that there is no need to be concerned about watering wisteria unless the leaves start wilting, interesting. They also coach against over-fertilizing, could this be the problem? One person that wrote in to an expert about a similar problem was cautioned that it sounded like the soil around it may have gotten hit by round-up or other pre-emergent herbicide. The only other thing is that all articles talk about the need to prune, which I have not done with mine yet and am kind of chicken to do it. Good luck, I hope you solve this. I've just had mine 3 or 4 years and it's beautiful when it blooms!

Could it be too much water?? Doesn't seem likely, but what type of soil do you have? I have not watered my wisteria at all this summer and it grows on the south side of the house in a small strip of earth between the house and driveway. We are talking dry. I've pruned several times (I'm training it to grow in a particular fashion) and it has bloomed on three separate occasions so far this summer.