When its time to trim them for the 5th time,don't.They have stretched too much,the brass has moved from the head area of the case to the mouth.You may also be able to see a ring just above the solid head on the case body.Or the necks will split when you fire them,not all in one batch will split at the same time.But when some of them do it you can be reasonably sure the rest will the next time.If you pressure is a little to high the primer pocket will get to big to hold a primer.If you can load at the range you shoot at take one case and shoot it til it gives up.this will give you a good idea of what to look for.