You're probably crossing your eyes, which reverses the 3D effect. You're supposed to focus your vision behind the pattern, not in front of it. I dunno. It's hard to explain in words.
Pretend you are trying to focus on something through the image, like looking out a window. The pattern will start to overlap and when fully overlapped a "3D" image will snap clearly into view.
Once you learn how to see them, you can't not see them. Even though I haven't looked at one in ages, I instantly picked up the images here. Technique that works for me is hold two fingers in front of your eyes until a floating "sausage" appears between them.