Shared Calendar Permissions keep disappearing
 

Outlook 2010 and exchange 2010 - A user has complained that he has a shared calendar and he gives permission to about 3 or 4 other users. Every so often these permissions are lost and he has to give the users back the permission to view(these do not happen altogether, could be one user loses the permissions at one time). the same users also share their calendar to the rest of them and the same thing happens as well with permissions been lost. Any ideas?
I know this might nt be clear so if you need to know more please let me know.