1. Are the cases new, and perhaps too soft? I'm not sure what to do here.
2. Get the calipers out and check all your dimensions. Bullet diameter within specs? Inside expander about 2 to 3 thou' under bullet dia. Base of bullet fits inside the mouth bell? (I fit them in by hand, rather than pop it on top the case and jamb it.)
3. Are you using smokeless or Black? If you are using Black in a single shot it may not even be necessary to size the case mouth if you are single loading right at the range. This doesn't apply if you have to carry loaded ammo around.
I have collapsed a few cases in my time, but not how you describe. it happens!