You've tried to record a program that uses video hardware acceleration. To get better performance, these applications bypass the usual methods of writing to the screen, and FlashBack cannot capture them.
There are generally 3 types of program that do this:
- Video players
You may be able to find a setting in the Player to turn off acceleration features, or 'overlays'.
Look in the game's video setting for a 'Rendering' option that you can change to 'Software' or 'Use video hardware acceleration' option that can be disabled.
- Web browsers
There should be a 'Use hardware acceleration' option that you can disable. The location of this option varies between browsers but you can find it for your browser with a quick Google search.