You need to flash them again using a different SD-card , if the led lights don't blink then it's unlikely your miner is going to work, therefore it will not be request an IP address from the DHCP server, I don't have a technical explanation on how this problem happens, but sometimes an SD card just won't work well for a certain miner.
Avoid newer cards, use the older, smaller and slowest mini sd card you can find. Don't use SDXC and preferably not SDHC, the smaller the better, under 1g.
I could mention that an old SDHC reader isn't ever going to read an SDXC card, for example, but there is more to it.
Another reason is the way you are imaging the cards. DD under Linux is always the best way, as Windows and osx are too unreliable. In any case do a
sync after
dd before ejecting.