Did you check your coil on an ohm meter?...maybe it's not making a good connection.
Sorry - just re-read your original post. It works on the mech, not on the Kbox.
Ok, you know that the Kbox will only fire down to .4 ohms, right? If your coil is below that resistance the box won't fire the coil. I don't know what wire you are using or how many wraps or any other specifics so this is really taking a shot in the dark.
Ok, just re-read your second post. I'm guessing that by the "default one" you mean the coil that came mounted on the RBA when you bought it. If that is the case, it still may be right at the lower limit of resistance (.4) or the Kbox is reading it low, and not firing. Can't possibly know without a resistance measurement.