Glass can certainly jam it. There is spot in the center of the bottom of the disposal where you can insert an Allen wrench to turn it by hand. Try turning it backwards to free jammed material. Then with power off, take a flashlight and look inside to see if there is anything in the disposal. You can use tongs or long needle nose pliers to remove glass or other obstruction. It should spin freely with the allen wrench. If it hums too long the overload will trip. You can reset it by pushing the red button on the bottom back in. GOOD LUCK! Let us know if this helps!