I'm having the same issue on only one team - my DKs. I've triple checked everything, synced all their keybinds.
Working like this:
- Toon 1 and 2 will assist each other, 3 and 4 ignore them.
- Running toon 4 as main, all four are in sync and assist fine. But only toon 4 will get them all to assist.

The FTL keys (left shift, ctrl, etc.) were auto-generated and are identical to my 90 shaman that have no issue at all.

Checked Jamba as well. Everything is normal.