I've also always had a hard time with these. I suspect it's because one of my eyes was slightly lazy when I was a child, so my brain learned to put more importance on the signal from the other eye. When I cross my eyes, the image from the better eye tends to just totally override the other one, so it can be really hard to see these kinds of effects.
I have exactly this as well. My optician explained it as my brain would use the information from the lazier eye only if there wasn't any information from the good eye. Just tried the eyes crossed trick on the easy image in the article and the 3rd image in the middle is the right one. If I let them drift apart so there are 4 images I can see the left one and the difference (because I'd already found it), but as soon as I force them to overlap the left signal disappears and I'm only seeing the right image. I've also never managed to do a magic eye or anything, and 3d movies just give me a headache.
I seem to have the same thing. I can't get double images or any tricks at all.
I tried putting folded paper between my eyes to divide the images in such a way that I would only see left side with my left eye, and right side with my right, and I can alternate between image from left and image from right, but I can never see the image at once, or right side when I'm using my left eye.
