Check if the Yahoo Mail app you are using is up-to-date. AN outdated application will result in issues like Yahoo Mail not syncing Android. To solve how to sync yahoo mail issues, clear the cache and app data from your Android device. This will clear the sign-in data, settings, and also the temporary files which further can resolve the syncing issue. Force quit the Yahoo Mail app on your Android device and then re-launch it. Once the active memory of the app is refreshed, you can check if the syncing issue is resolved. Uninstall the Yahoo Mail app and then reinstall it.