If your screen is badly cracked, cover it with a layer of clear packing tape to help the suction cup adhere. Alternatively, you can pull using very strong tape instead of a suction cup. If all else fails, try supergluing the suction cup to the broken screen.

Significant force is required on the suction handle—you don't need to pry hard with the Jimmy. If you're having trouble, apply more heat and try again.