I know where you are coming from. So many times I have been "so sure" I "knew" the answer but couldn't make the "answer" fit. Time away on a different xword and the answer generally just slaps me in the face. I am also lucky enough to have a xword savy husband who is a bulldog on the internet when I am stuck.
Yes, sometimes your mind just doesn't register another meaning
for the clue.
If I am puzzling at night and can't figure out an answer (usually
because I am tired), I put it away until the next day and the
answer just jumps out at me the next day.