Mobs not dropping custom items

#
# These example files show you some of the basic attributes and
# skills MythicMobs can use. Far more is possible!
SkeletalKnight:
Type: WITHER_SKELETON
Display: '&2Skeletal Knight'
Health: 40
Damage: 8
Drops:
- gold_nugget 2 0.5
DropsPerLevel:
- gold_nugget 1 0.5
LevelModifiers:
- health 5
- damage 0.5
Options:
MovementSpeed: 0.1
SkeletonKing:
Type:...