I'd come across this before, but only just now experienced it. It seems much easier to lock on to the hidden image when it's animated. Also panning your head up/down/left/right with your eyes seems to help too.
Even when your eyes are pointing in the right direction, you eyes' lenses can be incorrectly focused. In order to see the image, you have to hit the correct combination of focus and parallax (which will disagree with what your brain is used to seeing).
> Is there something physically different about my eyes or brain compared to people who can see them?
This is possible, but probably unlikely. If you can't perceive depth, e.g. neurologically incapable, or you have less than two eyes, this won't work for you.
Even when your eyes are pointing in the right direction, you eyes' lenses can be incorrectly focused. In order to see the image, you have to hit the correct combination of focus and parallax (which will disagree with what your brain is used to seeing).
> Is there something physically different about my eyes or brain compared to people who can see them?
This is possible, but probably unlikely. If you can't perceive depth, e.g. neurologically incapable, or you have less than two eyes, this won't work for you.