I haven't grown wisteria (yet) but from what I have read, they can sometimes be a bit stubborn about blooming. As there hasn't been much response here, I did a quick google search and found many articles under "How to Grow" + Wisteria. I read the first one that came up. This British link sounds right to me. http://www.gardenadvice.co.uk/advisor/plants/wisteria/
You can copy and paste this link or do your own search. I don't think it is as simple as adding a fertilizer. Hope this helps a bit. Ferne, Kamloops, B.C.