Using different versions of Outlook between user and their delegates can cause sync issues on ActiveSync devices like iOS (Outlook 2011's precedessor, Entourage was horrible with timezones too). See the following for a rundown:
How to troubleshoot missing and duplicate appointments in Outlook
- Minimize delegates
- Define standard process
- Don't move invites
- Use cal logging in Outlook
Considerations when you use the Delegate Access feature in Microsoft Outlook
Use same Outlook version (delegate-manager)
Outlook: Troubleshooting guide for missing, duplicated or unsynchronized calendar items (lots of good best practices)
- Always process meeting invites from the Inbox folder
- Process all meeting invites that you receive. Do not just delete the invite if you do not want to attend
- Use an "End Date" for recurring meetings
- Enable calendar logging in Outlook (2007 / 2010)
- Enable calendar logging on mobile devices (OWA -> Options -> Phone -> Start Logging)
Current issues with Microsoft Exchange ActiveSync and third-party devices
Organizer cancels 1 meeting, device deletes all recurring meetings: Update Rollup 4 for Exchange Server 2007 SP3 + iOS 4.3+
Outlook meeting requests: Essential do's and don'ts
- Try not to delete a meeting request outright because this is one way that meetings get "lost."
- Send cancellation and delete meeting
- Cancel recurring meeting
- Deleting request on one computer after accepting on another can cause meeting to disappear
- Always accept or decline a meeting request from your Inbox
- Don't move a meeting request from your Inbox to a different folder before you accept or decline ("sniffer" code in Inbox)
- If delegates involved, turn off automatic acceptance of meeting requests
