I just recently got the Master tier subscription as a gift, and I was really excited to be able to content-share with friends online for our shared game. I've had it confirmed with two separate friends, one of which has made purchases* on D&DBeyond, that the content sharing does not work through creating a campaign. Help?

*those purchases were for adventures, and I only own character creation tools like the PHB+Tasha's. Neither friend overlaps their online purchases with me.

Edit: I've enabled content sharing within the campaign, and made sure all sourcebooks are turned on (I own the players bundle and some bits and pieces from other books). Still nothing.

No, it doesn’t work through creating the campaign, it works through activating content sharing. You can activate it even for other people’s campaigns.

Right yeah, I get that. Sorry, should've elaborated.
I have content sharing enabled, with all sourcebooks checked, and it still doesn't work.

Have your friends "refresh" their characters by bumping them up a level and then back down. This can help make sure that their sheets "know" about the newly-shared content.