The key map does not need to be assigned to both the set and the individual characters. Assigning the set is the same as assigning all of the individual characters in it.

The duplicate Mapped Keys can be gotten rid of as Fenril suggests.
  1. Remove the second copy of fireball, frostbolt, and fireblast
  2. Either add a new WoW Macro Action to the list of Actions in each one, for the other character, or
  3. Add an Action Target Group called Mages (for example), put your mages in it, then for the Target, select Group:Mages
But, neither of these is the reason they aren't working. Here's a suggestion that might solve your issue:
  • The Key Combination you set for a Keystroke or WoW Macro Action does not need to match the Hotkey you press to execute the Mapped Key and thus there is a separate setting for it.

And in fact, if you are using keys like 1,2,3,etc and you have an action bar addon (e.g. Bartender, etc), those addons like to override the key bindings created by the ISBoxer addon. Change your WoW Macro Action key combinations to something more obscure, for example Alt+Num 1, Ctrl+Alt+Num 1, Ctrl+[, Alt+], and so on. Things you never hit, and don't want to remember (because you hit the Hotkey to execute the ISBoxer mapping, NOT the key combination to execute the WoW Macro).

I imagine you have an action bar addon and that's the reason they aren't working.

A few other things you need to be sure of:
  1. Make sure that the ISBoxer Addon is not disabled. This is in the Addons window from Character Select
  2. Make sure when you log in, you see a few ISBoxer Addon lines in the WoW chat window. It will say which Character it thinks it is. If characters are in a different order in your Character Set slots than you log in with, it's not going to have the right macros on the right characters