One other possibility..........Someone has swapped the trigger group for one from a Mini 14. When I was an active gunsmith I had a customer who owned both the Mini 14 and the Mini 30. His Mini 30 trigger started dropping out during firing. I did some investigation and couldn't find the cause. Then, I asked if he kept the two trigger groups straight while reassembly after cleaning. My customer didn't know. I called Ruger and asked if there was a difference. The service tech confirmed that while the triggers look the same, there is a slight sizing difference. I swapped the two and sure enough, the Mini 30 became very happy again with the other trigger group.
You just might have the same problem. ........... Big Cholla