On my test system, everything works perfectly... I get a callback
every time I modify my table.

On my production system, I never get a callback.

An odd thing I've noticed:  when running

     SELECT CALLBACK FROM USER_CHANGE_NOTIFICATION_REGS

on my working test system, the callback HOST= points to my
client computer.  This makes intuitive sense, since it's a
registration for a callback.

   (46, 4, 'net8://(ADDRESS=(PROTOCOL=tcp)(HOST=172.16.252.1)(PORT=49406))?PR=0',
    0, 0, 1800, 'MH.CBDEMO')
   # 172.16.252.1 = client box

On my busted production system, the callback HOST= string
points to the database server.

   (271, 4, 'net8://(ADDRESS=(PROTOCOL=tcp)(HOST=138.72.249.167)(PORT=50966))?PR=0',
    0, 0, 1800, 'MH.CBDEMO')
   # 138.72.249.167 = database node

So, two questions:

1.  Is it the case that HOST= should always point to the
     client machine?

2.  What's the best way to troubleshoot a problem with
     change notifications like this?

Many TIA!
Mark