I would try the following:
1. Open up 2 command line sessions
2. In the first session issue the update (should hang, waiting on lock)
3. In the other session issue db2 list applications show detail
4. Performa snapshot on the session(s) that are in lock-wait and find the agent that is holding the lock.
5. Force the agent off that is holding the lock, this may need to be done a few times if there are multiple agents holding locks on the table.