I recently set up a new campaign to run decent into avernus and purchased a few extra races for some of my players so that they could have a wider range of choice in their character creation but after getting the campaign set up and sending the invite link, none of them were able to access the races they wanted and I had to then create characters for them to join as. Not only that, but once they took over a character, any further modifications were restricted to merely basic rules options. 

Is there a backdoor way of enabling content sharing that might solve this issue or is there something I'm missing in setting up my campaign? 

Content sharing is a feature of the master tier subscription. Do you or one of your players have the subscription?

Yep I've had the master tier subscription for about 8 months now

Have you turned content sharing on in three campaigns? The maximum campaigns you can share with is three. You can check all of your campaigns here. 

If your Master Tier subscription is active, and you have not turned sharing on in three other campaigns, I would recommend opening a ticket with Support, who will be able to securely assist you with fixing any issue with your purchases.

Thank you!

ah, that seems to have been the issue (it was my 4th campaign as dm. have turned off for another campaign for now so. Thanks.