Besides the possible reason is that you have reached the maximum number of allowed stations that can be opened from the license, it can be due to the station either crashing or its process getting killed in the Task Manager which causes the Station to close but doesn't release the license usage.

What you need to do is open the Remote Admin Console->Stations-> look under each station to see what is the machine_name\user_account is still visible for the particular station.

Go to that machine and open the Station again and then close it properly, then it will release the license usage for that machine.