I've had the same issue. Things I've learned to check:
1. Check the crimp after you load a few rounds, I've had some not crimp correctly.
2. If loading with lead bullets, make sure there's not to much excess lead around the crimp area.
3. Keep your cylinder clean, just a small amount of build up where the round seats in the cylinder will cause this issue.
4. Buy a cylinder and moon check tools if you haven't already, these have helped out a lot.