My guess from your description is as mentioned above and that the threads were not tapped properly in a straight line down the axis of the bore. If that's the case, they need to be redone or new ones drilled and tapped by a gunsmith. You're not going to be able to do much on your own if that's the problem. I'd put it back together to take it out and see if you can sight it in as it is and hope the scope adjustments will take care of the error. If they do and you don't mind the scope being a little off center, you should be good to go. I'm not sure how much it costs now for a routine drill/tap job by a smith, but you may need to find out and have it done.