Training problem

I failed this problem: http://en.lichess.org/xcvcKyyy. Now missing the mate in one with f5 itself is arguably pretty stupid. The point is that there is a forced mate, but the problem is failed. Is this a feature? I was under the impression that I should get another try if I played an inaccurate winning move.

The "good move but you can do better" feature certainly needs more use and is one of my major problems with this tactic trainer.

Btw: this is the problem: http://en.lichess.org/training/19395, the link above is only to my wrong solution.