ok, crepe myrtles i know something about.
prune them in the late fall, or very early
spring. cut inward facing, crossing branches. DO NOT CHOP THEIR HEADS OFF! if you need to control how tall it gets, gently top off the top branches.
feed it well in the
spring and again in early to mid summer.
the brown tips on your leaves are probably due to not enough moisture in the air. in fact, i was surprised you have crepe myrtle in las vegas as they are really moisture & humidity loving
trees.
as for the lack of blooms, all bloom on new growth, but sometimes, if they are "settling in" or don't get enough food or water, they take longer to bloom. mine is 2 and didn't bloom either, but i'm confident that next year it will.