Ask Mr. Smarty Plants

There is a native bush or small tree, Malpighia glabra (wild crapemyrtle) that looks like a crapemyrtle but is not in the same family. It is also called a Barbados Cherry and occurs mostly in South Texas. Most of the crapemyrtles planted in gardens are native to Asia, and are selections or cultivars of Lagerstroemia indica or Lagerstroemia faurei (Japanese crapemyrtle). So, we're going to assume that is what you have in your yard. Here are the major reasons we found that prevent crapemyrtles from blooming well:

Crapemyrtles need at least 6 to 8 hours of full sun a day to flower well.

They are very drought resistant, once established, but flowering can be enhanced by irrigation during extended dry periods.

Trees that are continually fertilized with nitrogen will put on a lot of vegetative growth, but may not flower as profusely. Keep the lawn fertilizer, which is supposed to encourage green leaves, away from the crapemyrtles.

Low fertility. The crapemyrtle will benefit from application of complete fertilizer in early Spring (with a good shot of phosphorus in it) to produce enough energy for growth and flower production.

Heavily pruned crapemyrtle will put most of their energy into regrowing limbs and leaves and less energy will go into flower production. Please don't prune your plant with a chain saw; in fact, many gardeners advise not pruning at all.

Dealing with powdery mildew. Even before you pull out the fungicide, try thinning the canopy of the crapemyrtle (okay, pruning sometimes is good), to allow better air circulation. And, again, if it had more sun it probably wouldn't be quite so susceptible to powdery mildew.

You have probably diagnosed your problem correctly, that the less heavily blooming plant is in too much shade. The problem is, even when you plan ahead and put a new plant in a sunny area, the trees around it will grow, too. What was a sunny place when you planted it may be shady in a few years.