ps2 memory card failure to work

i bought 3 memory cards from different ebayers, put them in and successfully deleted previous owner saves etc but now they won't work. Have tried various things to no avail and when inserted it comes up on screen no memory card inserted. What's happened or what have I done wrong? HELP

Er, are they Sony brand cards? If so, are they counterfeits or genuine?

Have you tried to contact the sellers? Were the purchases under that "ebay protection" thing?