Scheduling and managing events using Outlook.com calendar
I too am having this problem.
I've narrowed down the problem a bit more. It seams the problem occurs when several
contact sources (i.e. Outlook, Skype and Facebook) are linked together. This sometime causes the
birthday for this contact to be excluded on the calendar. If I unlink the contact (in People) and then resync the Birthday Calendar, the birthday reappears.
In this case. The birthday was available in both the Outlook and Facebook record but naturally not in the Skype record.
Please solve this issue.