We have a crepe myrtle in a good sunny spot and we planted it almost 12 years ago, but it has never developed. It has barely grown and when it blooms, it only has a few scattered buds which bloom. Even the trunk has barely grown.
Crepe Myrtle Blooms
Certified GKH Gardening Expert
Have you ever fertilized it? It sounds like the soil may be weak and the tree may need nutrients. If you have never fertilized it, that would be a good place to start.
Also, is it getting enough water? Some soils do not hold water well and if it is not getting enough water, this could also stunt its growth.
Check the roots too. If something is damaging the roots, such as bugs or rodents, this could also cause it to be stunted.