For Divine Hero items set to provide bonus units, you need to log in to the game every time after 24 hours since the last time the bonus was issued. The countdown will restart if one of these items is unequipped or swapped.
If you equip 4, 6, or 8 Set Hero Items, but receive fewer free units than specified in the description, check if all items are unique. If you equip a duplicate of the one already equipped, this item will not be counted in the set.