at one of my customer locations. I have the SL1100 setup to ring department group 3122 at night, mode 2, then forward to the department group mailbox. The issue is that only one of the extensions in that group rings on inbound calls. No matter what I do it only rings one extension.

Have you set program 16-01-05 to "1" (automatic)?

Yes, 16-01-05 is set to automatic.

Need more info,
Is it working in day mode?
What type of trunking.
You said I tried everything, such as?

I think I figured out a work around for the issue. If I program a virtual extension to a button on each phone and DIL the line to the virtual extension all of the extensions with the virtual extension button rings. Good idea?

I've done it that way & it has always worked fine

If you are using Dept groups the extension cannot have any forwarding features set on them.