OS X :: Unable To Save Mail In Gmail's Sent Folder?

I've set up Mail to use Gmail perfectly, except when I send emails from Mail it won't save them in my sent folder, and it hasn't collected my old sent messages either.

I did a search and found a few people with a similar problem but that seemed to be attributed to the fact that they were accessing IMAP accounts. Anyone know what the problem could be with Gmail?

The only instruction I didn't follow in the instructions was that I told Gmail to keep copies of my emails in its inbox rather than archive them.