We have recently upgraded to Exchange 2013 from Exchange 2010.. We have certain users that have their calendar appointments disappearing randomly.. They do not go into deleted items, they just disappear...

Things we have tried / things to note:

• Recreate Outlook Profile
• Remove account from iPhone and recreate
• Copied their AD account and then created a mailbox on their same database to see if appointments would disappear with the test account (they do not).
• Removed all delegates for this user
• User's iPhone is on latest iOS (8.3)
• Removed devices that haven't synced in a while (old iPhone, iPad)
• It's not just one user, it's multiple users. The one thing in common is they are on the same database and they are in the same OU.. We are in the process of migrating one user to a different DB to test.
• These particular users have been migrated to the 2013 servers for months, however, the issue seemed to have start when we decommissioned the 2010 servers.

Anyone have any suggestions on what to check?

Thanks

Calendaring often is problematic when you have multiple fingers in the same calendar all working different kinds of devices and software at various versions and revisions.  See the below links for good advice on how to resolve these kinds of issues.  In particular, try to have just one person on one client platform making all updates to calendars.

http://www.networkworld.com/community/blog/exchange-outlook-calendaring-problems-lost-me

http://www.networkworld.com/community/ExchangeCalendaringProblemSolutions

Hi,
 
Basis on your post, I notice that certain users who have multi-client face an issue their calendar appointments randomly missing.
If I misunderstand your concern, please do not hesitate to let me know.

How about OWA?
Please try to turn off ActiveSync or remove problematic account in mobile device for testing.
Besides, double confirm the sync configuration in client and test the connection by run Test E-mail AutoConfiguration or Outlook connection status.
 
Thanks