I had the same problem with my first CORE 9L. From the local gun shop, cleaned and to the range. Very good and consistent groups, but straight left. As I backed farther from the target the grouping went farther left; it ended up about 6 inches left at 25 yards. I am not tooting my own horn but I am confident with my ability to know that the left-side patterning was not shooter-induced. I tried taking out the set screw and drifting the rear post just as the M&P manual and even S&W LEO rep said, but the rear sight wouldn't even budge. I ran to the gun shop, the owner/manager happened to be in whom I know fairly well and long story short, he took my old one and traded me for a new one. Cleaned it again, back on the range and the rounds were stacking center mass as they should. Thus, I would believe that it might be shooting left/low left if your solid and feel good about your shooting. As far as that sight goes, good luck; it's a tough one to move. Unless you are experienced with it, it might not hurt to have a gunsmith check it out or look at sight pushers, IMO. Good Luck!