I can't say that I've ever had any problems with any of the necks I've kept around long enough to adjust . I only use .009's on them too or light gauge on the acoustics. Most of those were mahogany too. I would think with a quartersawn neck that it would hardly need any adjustment. Perhaps there is a fault in the rod? Is it a stewmac one? One of the blue ones? You said you had the same problem on another one. Maybe you need to look at those two and see what's different compared to the other necks you made. Sorry I can't be more help. Maybe somebody else has experienced the same thing and will comment.