From the WoW Quicker Start Guide on the ISBoxer Wiki:

If certain toons will not follow certain other toons, and the ISBoxer Addon is enabled, it is usually due to a key binding in the game set to use Shift+F11, Ctrl+F11, Alt+F11, Ctrl+Shift+F11. 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+F11, then Ctrl+F11, then Alt+F11, then Ctrl+Shift+F11, and then unbind it. This will clear out any keys that would interfere with auto-following up to 5 toons.