As the various replies suggest, it could be one of several issues; only troubleshooting will give the final answer.
But I can give you my experience: My 7800 I recently purchased, sometimes it will apparently not work, but it's just that the cart was not installed fully. I can't speak for others, but on mine, the cart slot is a tight fit, and I have to work to make sure to push it all the way in so it functions. Whereas on my 2600, the carts go in easily and I do not have this issue.