have you used the computer for a different Apple ID?
Apple draws a fine line.
When you turn on iTunes Match or Automatic Downloads, or when you download past purchases on an iOS device or computer, that device or computer becomes associated with your Apple ID.
Your Apple ID can have up to 10 devices and computers (combined) associated with it. Each computer must also be authorized using the same Apple ID. Once a device or computer is associated with your Apple ID, you cannot associate that device or computer with another Apple ID for 90 days. You can view which devices or computers are currently associated, remove unused devices or computers, and see how long before they can be associated with a different Apple ID from the Account Information page in iTunes on your computer:
Sign in to your Apple ID by choosing Store > Sign In from the iTunes menu.
Choose Store > View My Account from the iTunes menu.
From the Account Information screen, click Manage Devices.
Next to each device or computer name, you will be able to see when each was associated to your Apple ID. You will also be able to track how many days are remaining before your associated devices or computers can be associated to a different Apple ID.