I've seen this happen and I have two suggestions.
1) Use different (maybe larger or better fitting) plastic adapters so they'll fit better and not dig into the frame when pulling tension. Are you pulling higher than the recommended tension?
2) If #1 isn't an option, use some sort of leather or replacement grip pieces to supplement the plastic adapters to give a secure cushion to the frame.
I've only had this problem pulling at high tensions or on old machines. The Prince P200 suggests you crank the racquet "out" and "tighter" after mounting the frame. It's possible to get over zealous and indent the frame this way. We all know that it is horrible to pull tension and pull the frame off of the mount but some of us go overboard in mounting to secure the frame and guard against this happening. I'd suggest you might've done that in this case.
Back to using POGs. Why did I ever leave you dear friend?