Sometimes this can be due to something interfering with the connection between the pins in the PCMCIA slot and the pinholes on the card. This can be caused by something as simple as a few specks of dust. Try removing the card, blowing lightly on the pinholes and inside the card slot
(preferably with some sort of air-blowing device like a blow dryer so as not to get moisture into those areas
), and reinsert the card.
