Are you using a roll crimp die?
If so, not much crimp or the mouth of the case will bite into the bullet, actually causing it to be loose in the case.
Crimping just enough to smooth out the case and allow easy loading into the cylinder should do it.
Use as little belling of the mouth for bullet insert will reduce the crimping necessary.
Walt Kelly, alias Pogo, sez:
“Don't take life so serious, son, it ain't nohow permanent.”