Hello, I found this info about linking your Yahoo account to your Gmail. Check that you've followed the steps. If you have already done this look at this article where they explain 2 further solutions.
One can link Yahoo mail to Gmail through various methods. Direct methods to do so are:
Import contact and mail messages from yahoo mail to Gmail
Add Yahoo mail to Gmail
Both these methods take time to get synced and get Yahoo mails onto Gmail account.
While importing contacts and messages from Yahoo, you need to perform the following procedure:
Get access to your Gmail account
Go to Settings >> Accounts & Import >> Import Mails and Contacts
In the new window, provide Yahoo ID and proceed
Click Continue again and you will be redirected to Yahoo login
Provide Yahoo credentials in the new window and Click on Agree for ShuttleCloud Migration. Close the Window.
Click on Start Import option to import all the Yahoo emails and contacts to Gmail.
The notification of Yahoo emails and contact import is shown at every Gmail login for the next 7 days.
Another method is to add Yahoo mail to Gmail and this can be accomplished through the following procedure:
Access Gmail and go to Settings >> Account and Import
Click on add a mail account.
In the new window, provide Yahoo credential
Click Link accounts with Gmailify and proceed
Log in with your Yahoo credentials and click Agree to allow Gmail to access Yahoo mail
Then the Yahoo mail will be added on to your Gmail account.