If your system doesn't recognize the additional Add-on cards then you should do following:
1. Make sure that the card is plugged in properly in the slot, if the need be please reseat the card and make sure that the card is configured for proper resources (if it's non-PnP card)
2. Try to flush the BIOS to see if that helps at all in detecting. (FLUSHING THE BIOS: Turn off the system. Hold down INSERT key and then turn on the system)
3. If the previous solutions do not help then take the card out and put it in another slot.
NOTE: If the card is detected in another slot then motherboard may be defective and needs to be replaced. If the card is not detected then the Add-on card may be bad.