Depending on a number of variables like where the pan and its parts originated from you may have a tolerance stack up issue or poor QC.
I would suggest trying a longer GOOD QUALITY 1/2-20 bolt to see how snug or loose it is in the pan threads. If it's too loose you could go to an oversize plug to resolve the issue. I don't like suggesting that but something is not right and it needs to be identified and remedied.