Acrobat pro 9 no longer will accept my Serial Number?

was your acrobat 9 part of a suite?

if so you may need to use the suite installation files.

if not, make sure your serial number (6 groups of 4 numbers) is for acrobat 9 pro and make sure you're using the installation file for acrobat 9 pro.

also, if your serial number is for a pc (mac) it won't work with the installation file for a mac (pc), Error: "The serial number is not valid for this product" | Adobe Creative Suite