Hi,
It depends who is implementing the trigger.
If the grenade is implementing the trigger, that could be an issue if all ennemies are within very close range or at the same distance as the grenade.
I would suggest that each ennemy implement this grenade trigger and see if it works better.
also, I assume that all your ennemies are prefab instance originating from the same prefab, else double check that indeed all your ennemies are properly set up to receive damage in the first place.
bye,
Jean