I'm having this exact problem with my Moment and the google calendar. When I first got the phone the calendar seemed to be syncing fine (a tiny bit delayed but nothing I would complain about - I was thrilled it worked!).
But NOW I'm finding that if I add items or modify items on my phones calendar, it doesn't sync to the google online calendar. OR if I add items or modify them online, it doesn't sync with the calendar on my Moment.
This really isn't good. I need to have my calendar up to date! EVERYWHERE!!! I share my calendar with family members and they need to be able to see whats going on.
UPDATE: I noticed that there is a setting: settings->data synchronization. Make sure your calendar (and gmail and contacts) is checked so that it will automatically sync with google. It seems I had unchecked this for some reason.