Apple Mail is not syncing completely with Gmail account I have a Gmail account. For years, it has sync'd very nicely with my Apple Mail folders. Now, several of my Mail folders are no longer syncing with their respective Gmail Label (folder). In some of these folders may show some of the emails stored in my Google Gmail account, but not all. Some other Mail folders are completely empty now despite the fact that their may be 100-150 messages stored in the respective Gmail Label (folder).

I have tried using Apple's "Rebuild" function in Mail, but nothing happens in the Activity window. Any suggestions?

MacBook Air (13-inch, Early 2014), OS X Yosemite (10.10.1), 16 GB Ram; 480 GB SSD
Rebuild does not show up in the activity window, even when it is doing things in the background. I've experienced this first hand in the past. So, give it some time.

However, rebuilding I think only helps rebuild the index file based on the messages stored on your computer. It does not compare notes with the server, I believe. 

Honestly, I would just remove the account and add it again, so that it downloads your entire account back to your computer. Not an elegant solution, but it has done the trick for me a few times.