This may be caused by a delayed refresh setting in Yahoo Mail. Yahoo Mail inboxes do not display emails as they come in, instead it refreshes every 10 minutes to display emails received during that time.
Although the refresh setting rate can't be adjusted in Yahoo Mail, there is a way to manually refresh. Check out the steps below to refresh your inbox.
Using a web browser on a computer:
1. Hover your mouse over “Inbox” in the left panel.
2. Click "Inbox".
3. Your inbox is refreshed.
Using the app on a mobile device:
1. Go to your Inbox.
2. Swipe down from the top of the screen.
3. A refresh icon will appear (circled in the screenshot) then lift your finger from the screen.
4. Your inbox is refreshed.