Hi emx, Has it become more shaded by other plants around the arch ? that could be one possibility.
Many wisterias are just starting to come into flower now, if they are positioned well (late may to early june is usual)
A sunny but sheltered site is best and it will also cope with partial shade.
It should flower in june and then again in august and is best pruned in between in july, citting back the current years side growths to approx 6in's
A mulch of compost around the roots (spring & autumn) should also help.
A slow-reliese pelleted fertilizer givenevery 6 weeks or as directed throuthout the growing season should keep it looking good. Good Luck Tbird+