I know what you mean, so frustrating
e.g: just spent so much time on zzdfa's new problem and didn't study ;(
Happens to me all the time that such a simple solution to a problem only comes after so much time. Fortunately for us I hope real life isn't about solving 10 questions in an hour and more about real collaborative thinking over relatively more relaxed periods of time so hopefully good but not machine-like thinkers like us will somewhat thrive in such an environment. I heard about this phenomena from a few anecdotes from mathematicians.
but as far as studies go, i'll say go over not only the mechanics of the questions but also the mechanics of the thoughts, e.g: what you associate with what.. make general associations between concepts etc.
Also sometimes I leave a question that I don't get till last and while im working on another one it suddenly pops in. I remmeber my teacher advising that, "read all questions and then let your subconcious work on all of them simultaenously", in fact it sounds like a quote from famous french mathematician Poincare.
The worst is when you realise 5 hours after an exam all the mistakes you make, happened to me :/