Vanilla Ghast

It seems the vanilla ghast can't be changed, i totally don't know why

I wanted to give him a skill to shoot multiple fireball in series for an instant, but nothing happen

Then i used a skill which work on my Wither skeleton and gave it to the Ghast, in the simplest way, and still the skill does not happen , see in my Vanilla config, i can't make something simplest than that to see if something work or not,

So where am i wrong ? Maybe beacuse he is already shooting in his vanilla self ?...

Vanilla Ghast