I've been seeing this question a lot more often than I've seen the answer, so I'll post it again.
This is a known bug in Pokemon Platinum. Not sure if it affects all copies or just some of them. Nintendo will repair it for free. Call the customer service help line and explain the problem. They will do some searching and then send you an email with a printable FedEx shipping label. I just sent mine in today.
By the way, this will wipe your data, so you might want to trade off your rares and starter first.
Note: this doesn't apply to the people getting connection errors and freeze-ups. It only applies to those of us who CAN connect to GTS, but CAN'T respond to trades even though you have the Pokemon they're asking for.
<big><big><big><big>WAY TO GO NOA!</big></big></big></big>
Gosh darn idiots.