Missing piece

[Don Giro]

I have done a substantial part of the game, but I'm stuck on what looks like it should have ben a very early puzzle to solve: it's the corridor door puzzler where you rotate the dial by 3 or 4. The puzzle says that I'm missing a piece, and I can't find it anywhere. I watched a walkthrough where the player approached the puzzle without the need for a "missing piece." What did I do wrong? Like I said, I've been through several time periods in this game, but I still don't have that piece...

[Potatoes Aughratin]

I'm having the same trouble. I've looked all over for a way to progress, but can't find anything. The hint button keeps telling me to use the time machine, no matter where I'm at.

[Potatoes Aughratin]

I had to get off of expert mode, and then a room opened up that I couldn't get to before. But from there I could go on.