So I have this problem, where some of my players can't see all of the subclasses that I have available. One player can only see one subclass. I have the master to your subscription. I did cancel all of my other campaigns except for this one.

Which, I shouldn't have had to do by the way, because the master tier subscription says that I should be able to have five campaigns active, and sharing content with them. If I can't in reality do that then D&D Beyond is guilt of false advertising.

When I look at the players character from my DM side, I can see all of the other subclasses. So why can my player only see one and how do we fix it?

Characters need to be made/edited in the campaign that has sharing enabled.

There are two versions of every core class, the 2014 version and 2024 version - ensure they're using the correct version to see the relevant subclass. 

You and the players should ensure entitlements are in sync by going to Account > Licenses , or click here: https://www.dndbeyond.com/account/licenses

You can try disabling then re-enabling Content Sharing to refresh it.