Certified GKH Gardening Expert
The tree may be suffering from an iron deficiency. This normally happens either when the plant is watered too much and the nutrients get leeched away or if the pH is too high. Test the soil and correct and nutrient and pH issues found when testing. Also, make sure that the soil is not too porous and is letting the water run away too quickly.
If you have not yet read it, this article may help with the blooming issue: https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-not-blooming.htm