I've had this problem as well, not sure if its a bug or not, this is how i solved it:
The clues to the positions of the symbols on the stones in Plato's lost dialogue are correct, but sometimes they dont have to be lined up with the Bull horns.
Try lining up the correct symbols in the '3 o'clock' position & pushing the peg, if that dos'nt work try the '6 o'clock' position or the '9 o'clock' position.
This usally solves the problem for me.
Let me know how u get on.