Mail not getting Gmail via IMAP

So I have a Gmail account, and I use IMAP via Mail to access it. When I click "Get Mail", I frequently won't get messages that I know are sitting in my Gmail inbox for some reason. I have to take the account offline and then back online to get my new messages. Sometimes it works normally, but most times it doesn't. Also, when I clicked "synchronize account" this used to work but doesn't seem to any more. I have another IMAP account using mail that goes to another email I have with my web hosting account, and that one always works perfectly. I also occassionally have problems sending messages through Gmail using IMAP and Mail. Anybody know what the issue may be? I'm using Mail v3. Thanks!