Some people’s vision might limit their ability to see the illusion, either that or they didn’t watch it long enough.
That is very true. Also, the ability to put the images together in the brain may have something to do with it. There are vision tests that can show if you have this problem. It’s the one where you tell the tester which number the arrow is pointing at on the horizontal test, if you’ve ever taken one at the DMV, etc.