They are ubiquitous in deep shady spots in the south, so unless it's a new cultivar of some type, I'd be surprised if sun was the issue. It sounds like you may have the type that blooms on old wood. If so, when the flowers are finished is the optimal time to trim for shape and size, then not again until it's leafed out. At that time in the spring, you'd only want to manicure, not prune, so you don't remove the flowers. If there are wispy twigs protruding from the strongly growing nodes, that's what I trim. Anything more and you've removed your flowers.