Check to see if there is a GMAPSUPP.IMG on the main memory of your GPS Receiver. If there is, then the file on your GPS receiver takes precedence over the file on your SD card.
If you do find a second GMAPSUPP.IMG, then make sure to back it up (and any other file that look related) before deleting or replacing it.
What GPSr? Oregons, Colorados, and Dakotas, have internal memory that will support more than one gmapsupp.IMG. 60CSX and older only permit a gmapsupp.IMG on the removable card.
What size card, and what format? While 8GB cards may work in some situations. 4GB is the largest card generally supported.
A poorly seated card will produce the results you are seeing, so make sure the card seats correctly. If it seats correctly, check that you can open the gmapsupp.IMG in GPSMapEdit. MapEdit will complain when opening a gmapsupp.IMG, but you should be able to view your map.