See attached screenshots. The piston is not powered by the redstone dust behind it even though the dust is pointing into the piston. The piston only becomes powered once you connect that piece of redstone dust with something else, like a lever/button/redstone dust/redstone block. This can be on the same level, below or above (if applicable). The piece of redstone dust may not be connected to anything on the side, otherwise the piston won't be powered either.

I'm not even going to attempt explaining what logic error caused this bug.