You may need to go into mail settings under forwarding POP/IMAP and check to see if IMAP is enabled. If this setting is off your account will not allow anything to communicate with it. If it is just your calendar that isn't syncing then you may need to open your calendar on your phone and double check what exactly is syncing in there. There are phone calendars, other account calendars, the one you want may not be selected. You can always delete all your account information and put it back in there.