Mail does not sync sent items from gmail

[Guest]

I use a Gmail account that I access from the web at work but like to use the Mail app on my iMac at home.  But I find that when I get home all of the emails that I sent through the Gmail web app do not appear in Mail's "Sent" folder.  It's set up as an IMAP account, just as my iPod Touch is and the Touch doesn't seem to have this problem.  What can I do to make sure that Mail mirrors the sent folders on Gmail and on my iPod Touch?