Mail and Gmail sync of Sent folder problem

I have had a Gmail IMAP account sync with Mail without problems for over a year. I almost never actually log into Gmail in a browser anymore because Mail works just fine. The other day I was away from my computer so I logged in via a browser and saw that about a month ago, Gmail stopped saving sent messages in the sent section. Those messages are still in my Mail folder, but never got sent to the server or not saved. I never changed any configurations.

Is there some way to manually "push" those messages to keep things in sync?

I have checked my settings, everything is normal.