I'm in 7a and winter my mandevillas over inside. I cut them back pretty far when I put them out in the spring and keep them in full sun. I give them Superthrive and B1 at the beginning of the growing season and feed them once every couple weeks with either an all purpose or bloom booster fertilizer (I try to alternate). I also give them epsom salts and kelp extract once a month. Now that it's so hot I've been watering them daily. They are loaded with buds.
Karyn

Well, I must be missing something. I wonder what it could be? I have fertalized with Bloom Booster and Miracle Grow, but I haven't used Superthrive, B1, or epson salts. Do you think one of those could be the key? Or maybe full sun would help. Any advice there?

I'd put in full sun at least until the buds started to develop. Mine gets full sun in a western exposure on a concrete patio and could not be happier (unless it drank beer).

Thank you. I'll move it.