Here is a list of possible scernarios, in which, a previously successful activation stops working:
1. After the installation you activate a 30-day demo, send in the request file and copy the authorize.mtk to the installation directory. This doesn’t activate the license (for that you have to use the licensing utility). Sometimes, it happens that you forget the last step, since you see that the software is running (30-day period).
2. If you use a VM to install the software on, you might want copy the VM to a different machine. In this case, you also change the hardware, so this can also invalidate the license
3. You are using a machine with redundant hardware components (as supplied by some manufacturers, for example Stratus). If a redundant part fails over, the hardware changes, this can invalidate the license.