The most common reason is that it isn’t receiving enough sun. Crape myrtle (Lagerstroemia indica) isn’t a plant that’s happy if it gets just morning sun, for instance. It needs to be in full sun at least eight hours daily.
Often a young crape myrtle is planted in full sun but over the years, nearby trees grow and we unfortunately discover what a difference a few hours of shade can make.
Another cause of nonblooming crape myrtles is improper pruning, or pruning at the wrong time.
Other than gradually removing lower limbs to create a tree effect, it’s best to prune crape myrtles very little. Plant them where they have room to grow, or choose cultivars that stay relatively small, if your space is tight. (Crape myrtles are also fairly easy to transplant from a shady spot to a sunny one.)
Finally, winter weather can damage crape myrtle, especially on the northern reaches of its hardiness zones. In spring after an extra-hard winter, you may think that your shrub or tree has been damaged beyond return. But don’t give up too soon.
Crape myrtles are among the last shrubs to leaf out, and will often surprise you by showing signs of life as late as in June, although they’ll skip flowering that year.
The National Arboretum, which has released some outstanding crape myrtles, has an informative site that will tell you everything you need to know about crape myrtles.: