I've had this working fine the past year, all settings are the same as they were when it worked:
Background data enabled
Fetch schedule the same
Notifications for display bar
Power settings are set to Max.
It seems just randomly last week I stopped getting auto sync/refresh and notifications of mail in my yahoo account. When I open it and manually sync(by pressing the red circle) it refreshes and I can see my mail.
This has happened to me a few times in the last year and in those instances I removed the Yahoo account, rebooted the phone then re-added it and everything was working again.
I did add my Google account to sync and this is working per the fetch schedule so I know there are no problems there. Seems to be just with the built in Yahoo mail.
Anyone hit anything like this?