Here is one typical problem you usually get when you tried to Stop DB2 subsystem. You just fired the command to stop the DB2 subsystem, but DB2 is not coming down.
After having a thought...you may assume there are certain active dedicated threads from a started task (DB2 related application or any job). The interesting fact is when you try to DISPLAY Threads through DB2 Panels to execute DB2 commands, it will give you a Message like DB2 Not Operational for the Subsystem.
Here is the trick:
If you know the Character for DB2 subsystem (You can get it from Zparms), you can fire the Display command from console.
For example in spool you can type