I recently purchased a saiga 20 and it is not ejecting shells. the bolt and carrier have been polished by previous owner. gas block is on properly, there are 3 gas holes in the barrel and all 3 are the proper size. I have tried multiple gas setting and still a fte every time. i have tried factory and aftermarket mags 2 3/4 and 3 inch shells. what could be causing this problem?