If anything,being potbound would make them bloom. I live in Massachusetts and have a couple of trees that have been in the same pots for 6 years or so. They get pruned and spend the winter indoors. Being in Massachusetts and you in Georgia my trees do not come into bloom until late August and continue to bloom until the temps drop to 50 degrees. Unfortunately this is all I have to offer.

Osmocote, super phosphate and Epsom salts. Mix one tablespoon Epsom salts per one gallon of water. Water once with this solution and again every four weeks. Use Osmocote per the instructions. It will fertilize every time you water. Also mix in a tablespoon of super or Triple phosphate once each season. You will see buds within 3-4 weeks. What kind of hibisuses do you have?