For example, I got completely stuck on the very simple ninja action hero puzzle where you had to follow the treasure map.
The problem wasn't figuring out the puzzle logic -- I figured that out right away. The issue was that for some reason my brain couldn't work out what the drawing for the start location was supposed to be, on the treasure map. To me it looked like a dancing house with bongo drums or something... I stared and squinted at it for a good 45 minutes before my brain finally started to interpret what that shape was.
After finally figuring out what it was, I now see every time I look at it. It's kind of like an M.C. Escher painting where focusing differently will make you see the picture in a completely different way. But it's frustruating to get hung up on a puzzle for a dumb reason like that though.