Shared Calendar Tracking: Only items in the default store....

Hi,

I have a user who tracks appointments from outlook to CRM on behalf of 5 other users. The user does this by accessing shared calendars in Outlook. She has full Delegate level access to the calendars. 

She is able to track with no issues on 4 of the 5. However for the fifth user, the Track button does not even appear on the appointment record when it is opened. The Track button does appear on the calendar view itself, but when clicked an error message is given:

"Only items in the default Microsoft Outlook store can be promoted...."

I've googled around and seen this is an issue with tracking email, but has anyone seen this kind of scenario with appointments? The user has identical access rights for all 5 users she is tracking on behalf of.

Help!

Thanks.