In the construction industry, we have found the stainless made in china (box store quality) is not a good grade stainless and will often attract a magnet. We have also found grade 8 and structural bolts from china fail all the time. They are not required to test like we do over here in the home country. We used to tork 3/4 and 1 inch structural bolts with an electric tork machine and the china stuff would fail with regularity. Not all fasteners are created equall. You get what you pay for. I would guess this is the source of yopur problem.