Sometimes a device such as a camera will use its own file system and you need to access the card in the device via a cable, rather than taking the card and plugging it into a reader.

Do you know what format the images are in? Many higher end digital cameras (especially SLR) can take images in both jpg and raw format. If they are in RAW then the format will be unique to the camera make and you need to install the software for your camera to use and view them.