Active sync issues with 8.3 - Calendar items missing
You can usually resolve this sort of issue by contacting Apple support and getting a debug profile/build to load onto one device and track down the iOS errors and correlate them with the exchange server logs.

Alternate methods would include using Xcode to look at the logs on the iOS device if you are lucky enough to have relevant logs in the main log. My guess is you'll need specific logging on both sides to troubleshoot and then address this.

While you're on with Apple support, ask if they have a current exchange configuration guide - often you can get suggested settings rather than having to systematically change things server side which is never fun even if you have a development server to test with. Testing in production is not something I like to even consider when you can get at the logs and know what's happening with a known amount of work.