Using Classic Outlook on Windows for personal email, calendar, and contact management
Hi Adam PT
Thanks for your post in the community.
From your description, we'd like to know how the secondary calendar created is. It is useful to help us to narrow down the issue. Meanwhile, based on the scenario that can't share calendar, kindly sign in Outlook web app to share this calendar and check the result on other users' side. For further assistance, there is the article about sharing your calendar in Outlook web app. Besides, to provide further assistance, could you please share the detailed steps about sharing calendar on your side? We're glad to help you to check it.
Looking forward to your update and we'll always be here for you.
Regards,
Joey