Do you check the logfile whether you see activities of your application code?
Can you call a SLSB after you initiate the shutdown?
I suppose the GC is not high active, this can be also a reason, but then the GC activity is also before you shutdown.
In the logs I can not see any activities from my application, it all seems to be like JBoss is stopping without any problems ... the only problem is the time it takes to stop.
I've attached some files created with jstack. Could you please have a look at them and tell me if you notice something suspicious ... I can see a : at java.lang.Throwable.fillInStackTrace(Native Method) which seems to take some time but in my logs there are no errors.