Unable to register Acrobat X Pro

Hi,

I have a new PC build and I have already deactivated Acrobat on my old PC.

When I try to activate Acrobat X Pro on my new PC I am receiving the error  "this serial number has already been activated on 2 computers. You must correct this problem to continue", "launch one of the other copies of Acrobat X Pro, click on the help menu and select Deactivate…"

I have already deactivated Acrobat on my old PC so why I can't register?

Thanks,

Asaf