licia, the following comes courtesy of my DH:
In case you had an older Yahoo mail account and you switched to the new Yahoo mail beta some time ago, you may want to switch back to the older Yahoo mail account to see whether this resolves the problem. This is so because betas are experimental versions of new products and therefore not completely free of bugs and hiccups.
If you are using the older Yahoo Mail account, it is better for you to ask Yahoo directly how to resolve your problem. From the Yahoo Mail page that displays your inbox, go to the upper left-hand corner to the 'Mail' tab and click the small triangle that faces down. In the small pop out window that opens click on 'Help." This will take you to the Yahoo Mail Online Support page. Navigate to the bottom of this page and click on the link 'How do I contact Yahoo Mail Customer Care?'. This will take you to a page where you can post your question to Yahoo and wait for their answer.
The proof of the pudding is in the eating!