I feel like this often. I think it is often due to the fact that our skill is usually different on different topics. For example, I am very good at greedy/implementation type problems, even if they are very high ranked. However, I struggle with problems that require more complex algorithms.

I guess that to a lesser degree our skill even within topics fluctuates slightly. Thing like sleep or even just mood can effect our problem solving ability.

Maybe because you are adapted to hard problems, you see a easy one and overcomplicate the solution, that happened to me some months ago, i was doing so much 1200 problem that i couldnt solve 900-1000