To be honest, it could be just about ANYTHING. The fact that it half-works implies a problem with a detection stage, and my immediate reaction is a diode the wrong way round (typical construction problem). I've built three of these units and they have worked fine each time, first time.

The only way to diagnose a problem is to dismantle the unit again. Starting with the first line of the instruction sheet, go through the instruction sequence from the very start and observe each component to check for an orientation problem. I am willing to bet it's a component the wrong way round.