Sent Messages not appearing in Mail

For the past several days many of my older Sent messages from my Gmail account have not been appearing in the Sent folder in Mail. The most recent messages from sent from Mail appear, but messages sent from other devices that used to appear in the Sent folder within Mail are not showing up. I can view my Sent messges within the Gmail IMAP folders and in my Sent folders in my iPad and iPhone Mail apps. What can I do?