Three summers ago I planted two dwarf Raspberry Razzle Dazzle Crape Myrtle. At the time I bought them they were blooming. The next spring I shaped/pruned them perhaps too early and I attributed the lack of blooms on that. This year I haven't touched them and still no sign of blooms. They do green up but no buds. I live in 6b zone and purchased them from a reputable nursery.
Pruning of crapemyrtles is not necessary to make them bloom. The only pruning they should ever receive other than for shaping is to cut out dead branches. Being up north I'm sure your plants suffer some winter tip damage. If you prune in fall then you may end up pruning again in spring because of winter die back.
Have you fertilized them at all? Could excessive nitrogen be the culprit?