Okay, then do one of the following:

1)
If certain toons will not assist certain other toons, and the ISBoxer Addon is enabled, it is usually due to a key binding in the game set to use Shift+Backspace, Ctrl+Backspace, Alt+Backspace, Ctrl+Shift+Backspace. You can go into the game's Key Bindings screen, and find a key binding that is not configured -- a blank box. Any of them will do. Set it to Shift+Backspace, then Ctrl+Backspace, then Alt+Backspace, then Ctrl+Shift+Backspace, and then unbind it. This will clear out any keys that would interfere with auto-assisting up to 5 toons.
Taken from the World of Warcraft Quicker Start Guide and modified for assist.

2) Reset your key bindings on every character.