In D&D 5e, Draconic Sorcerers, at level 6, gain the ability to add their Charisma modifier to one damage roll of any spell they cast that deals the damage type associated with their Dragon Ancestor.
Suppose a sorcerer has a Blue Dragon ancestor, associated with lightning damage. Can they add their Charisma modifier to Chaos Bolt, even if lightning damage isn't rolled in that casting? The spell itself can deal lightning damage, but not always.