This scenario is aplicable if you have multiple license files, say one original license file for 3000 licenses and you later purchased an additional 2000 licenses.
By default you will be issues these licenses as seperate licenses and most often you cannot use both at the same time. You can either make just the 3000 or the 2000 licenses apply but not 'add' them together within the Symantec Installation Manager (SIM) or similar.
The solution is to combine your licenses. You should be able to call/request assistance from Customer Care, but you do also have the option of doing so online utilizing the Symantec Licensing page.
First go to the Symantec Licensing page and log in with your account (this must be the account that has access to and is registered with the proper licenses).
Next follow these steps to create a combined license;