Outlook Delegates

[Mitchell]

Hi,

I have a number of mailboxes with shared Calendars and
Tasks folders. Some folders are either shared from within
Outlook under Tools-Delegates, some mailboxes are fully
shared under Exchange Advanced in Active Directory.

Every few weeks most, but not all, of these shares
disappear. That is to say, if I open a particular Outlook
client and view the delegates, some users are now missing
from the shared list, others may remain. Similarly, in
AD, if I view the Mailbox Rights, some names are gone also.

Can anyone shed any light on what is causing this?

Thanks in advance,
Mitchell