It will happens due to the following reasons,
- If the license key is already activated in another system.
- The Licensing Server looks for computer ID only, If the computer ID changed it won't allow to activate. The following situations can result in a changed Machine ID:
- When you've upgraded your OS, your computer ID has been changed even though the computer name remains same
- When the network adapter changed.
- When the motherboard has been replaced.
- When network card has been replaced.
- When running AccountEdge in a Virtual Machine (such as Virtualbox or VMWare).
- The way you connect to the network is a USB modem or express card. If you connect this way your Machine ID may only remain the same for that particular connection instance.
- On Windows some network adapters allow MAC address changing as one of the properties in the adapter's advanced settings. You can find these settings in the Device Manager.