If you are using an older device, such as a first generation iPhone or iPod Touch, you will not be able to rely on the calendar and should consider an upgrade.Update your phone/device software when prompted.
For details, see the Microsoft article at. However, to help work around these issues, the following is suggested: An exact cause is not always known and can vary from client to client. There are known issues that cause some email clients, especially older clients and those on some mobile devices, to provide inconsistent and unreliable updating of calendar information via Exchange ActiveSync.
