Some years ago, i asked here about a similar problem.
I had a Blood Fountain pretender that was maxed out in Blood magic. Because it was an endgame, and i wanted to test my best and well equipped commanders against some horrors, i casted the global enchantment "Astral Corruption".
To test my pretender itself, i launched an astral spell with it and the Slave of Unreason found its way to my fountain.
My fountain was scripted with "Cast Spell" to leave it free to chose its own. It casted only one spell, "Infernal Prison" that got rid of the Slave of Unreason.
But powerfull horrors like the Slave of Unreason never stay imprisonned for long. several turns after that, it appeared again in one of the province.
I sent my best commander, with the scripted spell "Infernal Prison" to see if he would do the same as my pretender to the Horror.
In the battle, he never ever casted "Infernal Prison". I didn't understood why, until someone mentionned the range. Infernal Prison has a range of 30 in the manual, and i had placed my commander far in the back, thinking about avoiding anything nasty the Slave of Unreason could launch first, letting my foot troops/canon fodder gets all the fun
So check if you fire mages are not by any chances too far from the target when it is time for them to throw the first spell you scripted them.