Some but not all contact birthdays are showing in the birthday calendar. Deleting the contact that's not showing in the calendar and recreating it from scratch solves the problem for that one contact but not the others. Some of the contacts show up in the birthday calendar and I can see no difference in the contact data for those that do and don't show in the birthday calendar.
I don't want to delete and recreate all my contacts to solve this problem. I've tried everything I can think of and spent many hours searching for a solution online with no success. I have sent a request for a solution to Microsoft but have not yet received a response.