Tactics Trainer question

Several times I have completed a tactic, and got it right, but there was no response afterward like a rating change. I think twice I have seen the same problem, and the same thing happened again. Today it happened, problem number 0539480, and so I tried a different move, wondering if the freeze happens every time or only with the right moves. It instantly recognized it as wrong and deducted the points. Has anyone else had that happen?

But how would the internet know it's the same chess problems? Today was the third time I believe when it was a problem that I had previously done correctly. Twice I have done the same two problems, gotten it right, and all four times it locks up. I've never had it lock up when I get one wrong and I've never got one wrong and had it reappear again later (that I know of). It just seems weird that some problems reappear but only if I get them right.