invalid serial number message

Hi.

Last year  I purchased Adobe Acrobat XI Pro, Disk version.

I installed it, and all works well.

However,

I am about to change from windows to Mac, and looked into the cross platform exchange / upgrade program.

When i log into my Adobe account - it does not show my Acrobat in the list of my products, when i try to add it, it says Invalid Serial Number.

Please could someone advise as to why I cannot get Adobe to recognise my serial number? I can confirm It is the 24 digit number on the back of the disk packaging, as specified by Adob, that I have entered into my account.

This is frustrating to say the least, and I really would not want to have to purchase the full software again just to swap to MAC.

I look forward to any advice.