Good question. Your MasterMine license number may have been "challenged" by another user, thus invalidating it.
In the "Launch Dialog" dialog, go to Preferences and "License MasterMine" to raise the licensing dialog. If you see "??" in the last box, do the following:
1. click "Users" to see which licenses are already assigned to other users. They each have a sequence number at the end (i.e. "-01").
2. Back in the Registration dialog, replace the "??" with "01" or another 2-digit sequence number that is no higher than the number in the left-most box, if someone else is already assigned license 01.
3. Then hit "Register" to finalize your assignment of this sequenced licensed number.