Q: Mail problem since upgrading to 10.10.5

My MacBook Pro is running on OSX Yosemite 10.10.5. Since a few days ago, I have not been able to see many of the (non-junk) messages coming via GMail, even though Mail registers parenthetically the number of messages received--for example, it may note InBox (6), yet only one or two messages are actually showing up in Mail. What can I do to remedy this? I can find the messages by signing in to GMail, but that's absurd.