I created a campaign and didn't see the content sharing button. Figured I had too many campaigns with it on so I went around and turned it off on all but 1. I'm Master Tier so I have up to 5 available. 

I went back to my campaign which has now had players join and I still don't see this button. I decided to experiment and I logged out/in, cleared cache on my browser, tried it in desktop mode, deactivated and reactivated the campaign, and even reset the invite link for giggles. Nothing worked. I then made a new campaign and NOW the button shows up. 

I don't understand why the button can't show up for a campaign already made with players joined. It should be there. In fact, it should ALWAYS be there and give a rejection if you try to use it after tapping out on all 5 content shared campaigns. 

I am having the same issue, 

i am master tier, i have less than 5 campaigns with shared content, a new campaign can allow content sharing, my other older campaigns can, just not the one i am actively attempting to run. All licences are synced, all characters have relevant content enabled.

It was initially able to use the content but for unknown reasons the button has just disappeared. very annoying after spending weeks prepping all the maps on the VTT. now it looks like i will need to start over from scratch.