Adhesion to the printed surface will improve over a 24 hour period
Use reasonable force when scratching the coating.
Apply a piece of tape to the coated sheet, pull the tape off. Does any coating come off with the tape?
Increase the intensity of the IR lamps using the control on operator panel
Slow the speed of UV coater down.
Consult with the UV coating manufacturer to see if they have a coating that has better adhesion characteristics.