Why are screenshots of my mobile game blank?

I'm developing a game for Android. So far I've been testing on an HTC handset and everything is fine. However, when I test on a Samsung handset - while the game plays ok, I'm unable to get a screenshot of the game. The resulting file is just a big black rectangle.

Similarly, when switching between apps on the Samsung, the preview image that appears of my game is completely black. The game appears when I select it from the app switcher, but there's some flickering as it comes into the foreground. There is also flickering when I switch away from it.

I've searched and I can't find a similar issue, which means I'm either doing something really obviously stupid or really stupidly weird.