Sometimes crape myrtles won't bloom if they are in too much shade, but if this isn't the problem, it is likely that the plant itself is simply a poor selection. Usually when this problem is reported, the crape myrtle in question is one that is not a known cultivar. It is often one that was dug up and moved from another site or was already there. If you cruise a quality garden center, the crape myrtles for sale will be labeled with cultivar names. These selections were chosen from seedling populations because they had superior characteristics, especially for plentiful and colorful blooms. Other seedlings were rejected because of poor performance. This illustrates the importance of buying named cultivars and understanding that quality plays a big role in nursery stock. A crape myrtle of unknown name and origin may never be a good landscape performer, and no amount of fertilizer or water can change that.