I have had nothing but problems with the new Yahoo Mail. I have 5 email accounts with them (don't ask, long story!) - they all got switched to the "new" mail.
I complained on one of them about the Delete button sometimes not working, and my messages not appearing, or disappearing when I am trying to read them.
here is what they told me to do, no kidding.
1. Disable any Internet browser's addons or plugins.
2. Temporarily disable any security software.
3. Delete your Internet browser's cookies and cache.
4. Update Flash Player.
5. Ensure that JavaScript is updated and enabled in your Internet
browser.
6. Restart your Internet browser.
7. Restart your computer.
8. Temporarily switch to a different supported Internet browser.
9. Restart your Internet browser.
10. Restart your computer.
11. Test from another computer; please include the results of this test
with any response to this email.
12. Test from another location; please include the results of this test
with any response to this email.
And I'm supposed to do all that just to read my email!! I don't think so.
Anyway, the good news is, one of my accounts somehow got switched back to the old Yahoo Mail. and I complained enough on another one, on Live Chat, and they switched me back!!