You may have noticed that calendar items more than 60 days old are automatically being deleted when you sync your BlackBerry up with Outlook, this is because your BlackBerry automatically is set to a 60 day retension, so fix this, follow the below process on your BlackBerry and it should stop if from happening:
1) Go to the home screen
2) Go to your ‘Calendar’
3) Press the ‘Menu’ button
4) Select ‘Options’
5) Select ‘General’
6) Change the setting ‘Keep Appointments’ from ’60 Days’ to ‘Forever’
7) Press the ‘Return’ key
8) Select ‘Save’
That’s it