As soon as they're done blooming and for a few weeks after is the best time, if you must prune it. I don't think pruning it will increase bud set though, but I don't know for sure. If you want to be really fussy, you can remove the spent blooms and that might improve bud set. You could fertilze it a little now too, with holly-tone or something similar.
Thank-You. I thought if it got more branches it would eventually bloom more.
It's just waning overall. When I first got it, it was full of blooms for about the 2 first years. Last year it didn't have as many blooms on it and this year there was about 10 or 12. Some branches didn't even get buds on them. It has continued to go downhill since I have had it. I think I had it too close to a Black Walnut tree. I moved it last year in hopes it would be better this year, but never happened. Maybe next year will be better. I'll prune after it flowers. It's very spindly looking and am wanting it to fill in more. Maybe it just needs a good fertilizing?
I bet it's just getting settled in, especially if it had been suffering from the juglone and then had some additional stress added on top when it was transplanted, it could take it a couple years to come back to its full glory.