Could be a number of things. The only two coming to mind are: "Do they need pruning?" and "Are they getting too much fertilizer?" Crepe Myrtles do better if pruned to their main structure. This helps "boost" the remaining branches. Also, Crepe Myrtles have a fairly low tolerance to fertilizers so you may want to hold off on that if that's the case. Good luck!
Happy Planting,

Are there other crape myrtle's in your area that are blooming??? How does the plant behave for you??? Does it die to the ground in winter??? Though they are generally hardy plants..., sometimes dying back, sometimes not, my experience has been that there is simply not enough summer heat and sun here in NE Ohio for them to bloom well... They need at least 2 straight weeks with temps above 85F to bloom well...