Get some good light and look into the card slot at the pins. A magnifying glass might help.
Look for pins that are bent over. They symptoms are of a bent pin. Bent pins in the camera body are a serious issue with CF cards, it doesn't happen too often, but when it does, it can be expensive. I've had a 50% success in fixing them, and my job involved engineering and testing things like this in a lab.
Being a high reliability company, we would not even consider putting a product on the market with pins in the power side of the interface. Power gets bent and touches ground or other circuit element, and the whole camera is slagged. If not, its just a few hundred to replace the card socket.
A really really poor design, but its worked its way into being a standard for many years now.
Thats why many cameras use SD cards. They might not be as fast, but if they fail, you toss a cheap card rather than the camera.