and giving it a good shaking, even tapping it some on your desk. there must be something stuck underneath it for it to do that. These things collect dust and pet hair like you wouldn't believe, and crumbs if you happen to eat at your desk. (like i do). if that doesn't help, do not be afraid to use a bit of force on getting those covers off. they were made to come off. That's how they were put on.
i end up buying at least two of these keyboards a year, but not due to your problem, but due to the fact that the letters wear off quickly, even with nail polish. good luck!