Calendar Events not Adding to Google Calendar

If I add an event to the ipad calendar it doesn't appear on my google calendar. If I add an event to the google calendar via desktop or my android, it appears in the calendar on ipad. If I delete an event on ipad calendar, it is removed from the google calendar. I've have searched everywhere and checked my settings numerous times. I just can't work out why I can't add events to the google calendar on my ipad. Any advice would be really appreciated - thanks!