Every now and then you might see a "Try Again" message when performing your review. What is it?

For endgame courses, soft fail moves are all the different alternative moves in a position that our 6-piece tablebase has calculated are equivalent and ok to play.

For tactics courses, soft fail moves are moves that also lead to checkmate or to a position of similar evaluation. They are calculated with a 1-point evaluation margin.

For opening courses, soft fail moves are all alternative moves in a position that can be played within a 0.3 evaluation margin. After a long time of testing we settled on 0.3 eval because it includes a lot of human moves recommended by authors, but if we lower the margin, then it often only includes "engine" moves.

The margin is calculated from the best possible move in the position, so if the best move is evaluated at -0.2, and the margin is 0.3, alternate moves can be considered up to 0.50 (half a pawn).

We use a strong chess engine on a dedicated computer with multiple cores. There is no specific depth but the engine is given a fixed amount of time to 5 seconds per position on 16 cores. If you see an opening move that's not a soft fail move but the browser engine is telling you it is, chances are it's a decent move, but our chess engine did not determine it was strong enough. Sometimes, as strong GMs do, the chess engine could need up to a full day running to reach a final verdict (e.g. 0.31 vs 0.29).
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However, since there are many courses on the site we cannot allow so much time for evaluation. We offer the strongest possible evaluation for every move and apologize in advance if we have missed any. If you have any feedback on this function, we'd love to hear from you.