There's some graphics mod for guild wars 2 that will interfere with game capture. Basically the graphics mod uses a (improperly constructed) proxy d3d9.dll into the game directory to intercept and redirect graphics stuff so it can do its magic. Unfortunately, do to the fact that it's improperly constructed, it prevents game capture from working. So you have to delete the DLL to make sure it works. If the mod does not have an uninstaller like a good mod would have, then you may just have to scour the directory trying to clean that mod up.