The recommended maximum glass thickness is approx 9.5mm, so this explains why it could not work on 12mm. I've personally tried on 10mm and it works fine though. You could try to remove the rubber gasket from the wet side to see if it helps, but I wouldn't recommend it as it may scratch your glass if you are not careful. Also, if you plan to run it at maximum speed, vibrations may cause it to drop too.