Grimrock

MAGES ENTRANCE Threw a dagger at the skulls head and it sent a fireball at me. At that point I didn't know about sending one back at it. Went on to do more of the game and even went down to level 7 and level 8 completing a lot of the game. Went back to revisit mages entrance but because I have already thrown a dagger which is lying on the trigger it will not send anymore fire balls at me. How do I overcome this or is this a glitch that will not allow me to complete this level...help!!

You don't need it to trigger any additional fireballs to solve the puzzle.