So I need to give Ysolda a mammoth tusk in order to marry her. However, I cannot get the quest from her for some reason. I just completed the Night to Remember quest, as well as the Ysolda's Shipment quest, but she won't give me the quest.

What do I do?

Do you have any other rare item quests active?

I actually realized that just a few minutes ago. I scrolled through my huge backlog of miscellaneous quests (this is on an older save so I can marry Ysolda instead, so I didn't complete all these quests) and found that I needed to get The Mirror for Captain Aldis. Went back to Whiterun, found Ysolda, and it worked.

Silly me.