Hi Andy,
Since the license server assigns the licenses randomly from the available pool, regardless of its subscription state, you can get licenses errors if licenses with expired and active maintenance plan are mixed on the license server.
To "update" any of your licenses, please open the License Manager (Start > Itoo Software > Forest Pack Pro > License Manager), click on the Add... button and paste the corresponding license code (not the serial number). This will update the maintenance plan end date.
Best regards,