How to Get a Specific Mob's Head to Drop

Kill the Mob Yourself

Just Mob Heads only rolls for a head when a player lands the killing blow. If a mob dies to fall damage, fire, lava, another mob, or a farm's automated damage source, no head will drop. This means you need to be the one who deals the final hit. Bring the mob's health low with whatever method you like, then finish it off with a direct attack so the game registers you as the killer.

Check the Config for That Mob

Every mob has its own drop chance entry in the config file, and some are set much lower than others. If a particular mob never seems to drop its head, open the config and find that mob's line. The value there controls its drop rate independently of the global setting. Raise it temporarily to test, or set it high if you want a reliable farm. Remember the config is only read on startup, so restart after editing.

Use a Looting Sword

Looting does not change whether the mod rolls for a head, but it is still worth using a Looting-enchanted sword when farming heads because it speeds up kills and improves other drops. Combine a Looting sword with a mob-specific spawner or farm, stand where you can hit the mobs yourself, and swing until the head appears in your inventory. Heads are normal items, so they will drop on the ground and can be picked up like any other loot.