Breech closing issue

[Pa SxS]

I had a 12ga. Parker that did the same thing. I believe the rim needs to be deepened slightly. Examine shells that work and shells that don't. I found that some companies shells had a slight radius when you look at the side wall where it meets the rim. Those wouldn't work or were very difficult to close the gun on. The other shells that had a more 90 degree angle would fit in the chamber just fine. The solution was to deepen the rim cut.