[tjhands]

Here's an easy way to tell if it's a matter of not belling the case mouth enough to allow proper bullet seating: Take a case that you haven't yet tried to seat a bullet in - but one from the same batch that you had previously considered ready to seat. Now take a bullet in your fingers and see if you can get the bullet to stay in the case by using only light pressure with your hands. You want the case mouth to be belled (opened) just enough so that a bullet will juuuuuust stay in the brass from using light hand pressure, but not so much that it can be pushed in to any real measurable depth.
If you can't get the bullet to stay in the case mouth at all with light hand pressure, you need to turn the belling die in a bit further until you get the bullet to stay in place. Then you're ready to seat it fully with the seating die.

That may be the problem, but.....maybe not.