There could be several reasons for this. It may not be receiving enough water to bloom correctly. It could also have a pH issue, which can be solved with dolomitic lime. If this does not fix it, then I would recommend fertilizing with an all-purpose fertilizer.
Here is an article with more information: https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-care.htm