I often run into people who have a hard time completing their math homework; usually, when I ask them what's going wrong, they say that they don't know where to start tackling the problem. It's not really their fault because sometimes, the same thing happens to extremely hard-working individuals; I've seen a few ambitious straight-A students take much more time on a homework problem than needed.
When you don't know where to start, it could be simply down to your brain refusing to function at that particular time. You may need to work harder on understanding the course material for that particular day; extra time often does help in that respect. But in the majority of the cases I've seen, the culprit is a lack of mathematical intuition, something that even the best students can be guilty of. If a student doesn't have this intuition, he/she could lack the inspiration to tackle the problem.
A natural feel for numbers improves performance in math class, just like how a natural feel for a basketball improves performance on the court. In sport, this could be considered the killer instinct, and it can be developed.