Ok...even though I said it most likely wasn't the hanger, stand behind the bike and have a look at it. Obviously it should be vertical and not angled in towards the spokes. After that put it in a work stand and undo the cable. Pedal it. If it shifts down to the 11 it was a cable tension or cable/housing problem. If it doesn't go down it's probably a limit screw issue. Pull on the cable and make sure the shifter is in the high gear position. Attach the cable and all should be good.
Which D/A is it? 9000? 9100? Rear derailleurs are silly easy to adjust and most problems (if looked at in a logical fashion) are easy to figure out. Don't always try to adjust right away...have a look and try to figure out what's not happening and why. Never be shy about going back to step 1 and redoing everything.