Try over adjusting the "correctable" side, then recalibrate the camera to focus that side again. Then, try to get the "uncorrectable" side to focus in. Sounds like you might have reached a focusing barrier of some type. I know with my Acti's, I have to adjust within a range.

Otherwise, has the lens ever been cleaned or dis/re-assembled that one of the lens rings may not be snug which can allow that lens to shift from perfectly perpecular? You mention it being older, maybe it needs cleaning if it hasn't been? Obviously this line of thinking could expand in either direction.