setModifier() does not work ?

I use setModifier() function but does not work. whyy ?

 

public function onDamage(EntityDamageEvent $e){
if($e instanceof EntityDamageByChildEvent){
$child = $e->getChild();
if($child instanceof SnowBall){
$e->setModifier(3);
}
if($child instanceof Egg){
$e->setModifier(4);
}
if($child->y - $e->getEntity()->y > 1.35){
$e->setModifier(10);
}
}