Has the stock been removed at some point for cleaning or other issues?
I would remove the stock and check for any binding or foreign objects interfering with the mag box. It sounds as if the stock isn't sitting correctly on the action.
I had a similar problem some years back with a twig being jammed under the stock, it would move with recoil and jammed near the recoil lug, caused all sorts of issues, didn't discover it until arter loosening and tightening the scope rings and bases, checking the floating etc, then finally removed the stock and found it! It rrove me insane having a rifle that shot clover leafs and then suddenly wouldn't group into 4"!
Anyway have a look for any binding ANYWHERE especially with the follower where it rides inside the mag box, have seen a rifle go from functioning fine to abysmal from just the floorplate being slightly twisted from being knocked. Also look at the floorplate hinge pin, it may not be seated flush, causing the floorplate to be misaligned more on one side than the other.
Cheers.
