Mail does not fetch all of my Gmail

When I started the Mail app this morning, it said "downloading 11 messages," but the messages never actually appeared in Mail. They were not deleted or sent to junk, they just didn't show up.

On the Gmail web interface, I found the unread eleven messages in my All Mail folder, (not in the inbox). They were not marked junk mail.

This problem is sporadic, because sometimes one or two messages will get through. When I do a "test mail" it goes through immediately. This has been occurring for a few day now. I haven't changed any settings.