I use hotkeys, a different one for each slot (alt num1, alt num 2, ... alt num5). I've removed everything and configured it again through the wizard and running into the same problem: when changing to slot 5, the others won't assist him. The only thing I can find are the FTL modifiers ...

slot 1 - L Shift
slot 2 - R shift
slot 3 - L Ctrl
slot 4 - L Alt
slot 5 - L ctrl + L shift

Note that I do not have 'Use FTL modifiers' ticked under advanced for each dps keymap (I googled and that was the solution for someone with a similar problem).

Now I do have an actionbar in wow keybinded to shif+ctrl+numpad 1, shift+ctrl+numpad 2, .....

What happens when I'm mashing any dps button when I have solt 5 as the one the others should assist: the others will try to execute a spell on that action bar which is bound to ctrl+shift+numpad ... (spells like drink / eat / mount or on that actionbar, so I don't have any FTL dps keymap that references to that action bar.

So long story short I think the FTL modifier key for slot 5 screws it up. Not sure if I break anything while changing that, but I guess worst case scenario is that I'll have to setup isboxer again, so worth a try.

Link to my isboxer profile