Hello, Yahoo has a 'save sent mail feature'. It is possible that this has been turned off. To turn it back on, or check if it is on, go to Mail Options > General Settings.
From here, go to Sending Messages, then click the box next to 'Save a copy of the message in the Sent folder', and click Save to keep these settings.
Now you should be able to see sent mail in the sent folder. If you are still having this issue, have a look here for Yahoo's guide to find missing mail.