On a Gottlieb Bank Shot (add-a-ball), when the score is achieved to award a WOW, the knocker sounds & then gets "stuck"...humming noise occurs. When ball drains, the knocker returns to rest position. Interestingly when a WOW is awarded for anything other than score value the knocker functions properly. Any ideas as to what might be causing this "sticking" and what is the solution? Thanks.