Exchange Calendar Sync not working all the time

Hi there,

our customers got several IOS 9.3.2 devices (iPhones and iPads) connected to a Exchange 2010 Server. We get a lot of feedback from them, that not all calendar items are synced to all devices. So sometimes a user gets the calendar entry on his iPhone, but not on the iPad. The Exchange Server has all service packs and regular updates installed. Any hints?

Best regards,

Stephan