Extra P might help Charlie, but the secret to 'forcing' a Wisteria to bloom is prune..prune..prune. Plants in general only take up the nutrients they need...not what is provided...more will only go to waste.
Early spring and again mid winter is the prune times.
Seed grown, they can take anything from 5-15 yrs to flower. I've heard of a bonsai in a pot that took 20 yrs.
Grow it to how big you want it, and keep trimming it back to keep that shape.
When its just growing, it puts all its energy into growing...not flowering.
Also...don't trim into the hard wood....leave about 5 buds on.