OK, first crape myrtles need an acidic soil. As someone said check your pH. The cheap Home owners stick in the ground will be OK for this. If the indicator pegs then add some of the Miracle Grow for acid loving plants (Blue Gray Box). You might add a little pH lowering sulfur. Ask your local better than average greenhouse keeper. Secondly your plant is a heavy feeder of phosphate. Buy some Fertilome Triple Phosphate. Dig a hole or trench around it and fill it with at least two pounds of this stuff. It will take all winter for it to become activated in the sand. Flowering trees and plants have got to have this. Here in Tennessee Wisteria plants have got to have this done every fall and have their surface runners cut for any flowers to appear in the spring, Do these things about September-October. Trim back those long flowering twigs. Crapes bloom on new wood. Don't give it the monkey paw effect but do trim this years growth back to the pre season growing pattern. Remember blooms on New Wood so let it send out new shoots to bloom.