I think he means the GPU frame capture button in the Xcode debugger.
For what it's worth, I've been having similar problems and I don't know what the problem is. It seems to work on the rare occasion for certain projects or devices... it's really frustrating! It's only the last 3-4 months where it's been a problem. Since then, I've upgraded iOS, OS X and Xcode. It could be any one of them causing it.