Question
typed in wrong email address  [email protected] wrong --  right email is [email protected] -- sorry, hope this doesn't mess up you sending me an answer

Answer
Here is a duplicate answer in case.
I would recommend that you prune them. Crape myrtle flowers grow on the seasons new growth. Pruning results in an abundance of new shoots that form flowers. It also reduces the amount of vegetative growth and funnels the energy of the plant into new growth and flowers.
Here is a web link that discribes how to prune the tree.
http://pubs.caes.uga.edu/caespubs/pubcd/L331.htm