I was having the opposite problem on my Samsung Stratosphere: events on the phone would sync to Google Calendar online, but not vice versa. I went through a lot of checking and rechecking settings, etc. What finally solved it was to do Home > Menu > Settings > Applications > Manage applications > All, and then select Calendar storage (not Calendar). I then tapped Clear data (which was at 0.95MB), and did a Power button Power off, then powered back on. The calendar synced both directions immediately. Details are in my post here (with links to Google forums): https://community.verizonwireless.com/message/788338#788338.

If you decide to try this, be aware of two things:

1. You'll need to check your calendar settings afterward, to see whether settings for syncing, calendar, time zone, etc., have been changed.
2. More importantly, as noted in the Google forum posts that my other message links to, after you do this operation events more than 30 days before the present date and 1 year after it are no longer synced to the phone. They remain present in the online Google Calendar, but not on the phone; you can only get them to re-sync to the phone one at a time by editing them online. Recurring events seem to be something of an exception to this; I found they were preserved backwards about 12 weeks and forwards indefinitely.

I hope this is some help; or better yet, that you find a less drastic solution than this!