are you sure your battery is in your laptop all the way? sometimes if your battery isn't properly placed, it won't charge when plugged in.
i only recommend checking because this happened to me once, but it was a good thing.
i purchased a used iBook (a few years ago) on eBay and the guy had mentioned in the listing that the battery was completely dead therefore it couldn't be charged. well, i just figured i'd get another adaptar and battery down the road, so i got it for a really cheap deal.
when i received it, i noticed the battery door wasn't closed securely so when i opened it, the battery wasn't even sitting flush properly and as soon as i popped it in, voila! it started charging. can't believe he never noticed this (i also think it was from a school, too).
check that, and if that's not the case, the only thing you can do (unfortunately) is go back to apple. if this is your second adaptor in a matter of a few months, it may be your computer/battery that's burning it. don't know why, but it could be possible.
since it's still under warranty, take advantage of it.
also, you may have a much better response if you post directly in the apple forums. whenever i have an issue with my macs, i always post there since those people are pretty much apple experts.
http://discussions.apple.com/index.jspa?categoryID=1
