A book isn’t being shared with my campaign.

Hi! I purchased Mordenkainen Presents: Monsters of the Multiverse, and I’m also subscribed to the Master Tier. However, the players in one of my campaigns can’t access the content from that book when creating their characters, even though content sharing is enabled.

Do you know why this might be happening?

Make sure they're actually creating the character in the campaign, and that "5e Expanded Rules" is checked in the Sources section of the Home tab of the character builder.

Also, both you and the player should go here and make sure your entitlements are synced.