Quoted from Bigun:I made an earlier post about the credit knocker in Firepower potentially being bad, however, after running through diagnostic mode.... it fires perfectly.
So, I can only conclude that there's conditions that need to be met during gameplay in order for it to fire.
What conditions are those?
Match
High score broken
at end of game Fire power mission accomplished and 3 X knocks will be sounded.
Game must be moved off free play
Credit Knocker is disabled when "Number of credit" Setting 18 is placed on 00