Double check to make sure you are checking the right country and there are no typos in your input.
If it still isn't working call Apple. Assuming your machine is still under warranty they will ask you to fax them a paper with all the pertinent info and a copy of the receipt.
The serial number checker will also show status for out of warranty and machines replaced under applecare (as in the original serial number) but I don't know if it will show the status of the machine it was replaced with without additional input from apple.