R4 chip

Question

HI i have recently purchased 4 r4 chips with memory and software already downloaded, i have no problem with 3 of them but with the 1 the ds isnt recognising that the card has been inserted, i have tried switching memoy cards and that doesnt seem to be the problem the problem seems to be with the chip.
I have tried contacting the company where i purchased them but they are not being very helpful. would be grateful of any suggestions as it is getting very close to xmas and all i need is one unhappy child. thanks