What kind of card and which camera model? Some older camera models have trouble with newer CF cards. And some newer cameras have trouble with older CF cards. Also, what is the card size? Some older cameras wether SD or CF can't use cards larger than 2GB or 8GB unless formatted on a computer first. My 20D can only use 4GB CF cards or smaller, unless I use 8G or 16G cards formatted with a computer, they'll work.
If you can tell me your specific camera model and card size and model, I can give you a clearer answer!