Cracks in the stock - if you can take it out of the stock and check it all over around the magazine and tang. Apparently between the recoil lug and magazine they can start to go? Well, not sure if i got that right but its somewhere in that area.

If you have dummy rounds perhaps check if it feeds .308 too, because IIRC they are sensitive in preference? Don't quote me on that one. I haven't shot mine yet!