Three things I can think of...

A) The macro has the wrong reference to target a mob. If the working two toons assist off the non-working toon, the cut/paste macro might cause a target issue if the non-working macro isn't taking into account its the 'main' / 'target master'

B) Keybinding isn't set correctly. If you press "F" for fireball, but one toon has "F" bound to the wrong spell (correct spell macro/icon isn't in the right spot) then you might be trying to do/cast something you're not intending and causing an error.

C) If you're using any form of /click, make sure the target for the click is in the right spot (left bar vs right bar, etc).

Usually when things bug out for me, its the macro with a typo. If you cut/paste, make sure that you got the whole code & you pasted over all the old code (leaving the first or last characters of the old code with the pasted new code would be screwy).

If its none of those, then start over with the basics. Can you type assist and get the correct target. Can you drag the spell from your book onto the bar & cast it (1 - 2 - 3, etc). If you want a keybinding, can you make it work with any spell (buffing spell like INT for example). Same with /click. Did you even get the spell from the trainer (Rank). Figure out the point where things stop working and you should be better able to fix the problem.