Yes, it's the tap. I was on this level for days before I accidently tapped the screen and the penny dropped.
Almost finished the free version now.
Some said he doesn't get it yet finished all the levels? Kinda bizzare, me thinks.
My 70+ year old mother told me she was up until 1:00am playing AB on her iPod touch. What's there to get? It's just something fun to do.