I was having this same problem, and after googling and reading your post, I was very angry. I was discouraged from even trying anything else. I figured we were toast. I had been so used to the old way of pressing power + volume down, and getting the timing exactly right, I thought I knew how to do it. I read the instructions and understood them.
But I had a breakthrough. I just hit it like a ninja. The power button and home button, I made sure they were struck in exact perfect timing, at exactly the same time. Not like CTRL+ALT+DELETE, one after another. But like you're trying to poke its eyes out, or apply that judo pressure point trick on the sides of its neck. I pressed them at the exact same time, and held them. It captured the screen shot.
I'm truly giddy that it worked, thus the childlike joy and the judo and ninja references.
Galaxy S2 (Epic 4G), 4.1.2
Could it be that you were trying the old way of Power + Button, but didn't strike them at the exact same time?