Outlook 2011 : cannot move certain calendar items ...

Hello, I have imported a pst file in outlook 2011 that contains calendar items. I then tried to move all these calendar items to another folder (called Archive). Some of these items were properly moved but I remain with 800 items or so that wont move... I didnt identify any specific commonality between these items. Some are meetings, some are appointments, and they're spread throughout the years... What is happening, and is there a way to move these items to my archive? THanks

I am on Snow Leopard, and have the latest office 2011 upgrade