The license activation is based on the Physical address of the active network card.
If you have both a hardwired and wireless net work connection and switch between which one is used this can happen.
Some VPN Packages create a Virtual Network Adapter each time it is started. Each time the Virtual Network Adapter is created, it will be given a new Physical Address which will cause activation issues.
In some cases, on Mac OS the activation wont stick if there is only one computer user. If you are using MAc, try creating a new computer user, you will only need to login as that user once, then log back in as your regular user, this can sometimes fix issues with activation sticking if it is not caused by networking issues described above.
