i had this problem the power cord may seem as though its plugged in but sometimes needs a bit more of a push and then the GT light should light up if thats lit it should work, otherwise it just runs off the ps3 and only powers the buttons
Lol when I first tried my G27 I was perplexed as I had the same problem & I couldn't avail warranty as I got it from another country. But then I realized what a silly thing it was