Just yesterday, my grandson (19yo) helped me reload some 9mm.
I noticed that when he was belling the mouth, he was not seating the case firmly into the shell holder (not slid all the way back). This resulted in a visible 'bulge' to one side of the case. Things did not improve much while seating the bullets.
I let a few go and made sure that we used those as we stepped out to the range.
They, as expected, did not fully chamber and required clearing by hand.
He now understands the need to make sure everything is done as it should be.