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It might be helpful to know what device you have but honestly I've had issues with the Gmail app in general myself. Because of these issues I've moved away from it and tried to find alternatives. Before switching to Samsung I was using Maildroid. It has a lot of useful features, and at the time it was one of the few that allowed you transfer to a new device without having to go through the set-up process each time. It also did this after factory resets. Since moving to Samsung, I find the Samsung Email app to be a good alternative. You can view all of your emails in one view or individually in their own view. You get access to the spam folder for any of your accounts easily, and overall I've never struggled to add accounts. With other apps you sometimes have to change or tweak settings for sending or receiving emails.
One other thing that also pushed me away from the Gmail app was the ads. The last time I used it there were banner ads mixed in with your inbox.
Here is a step by step specifically for adding Yahoo to the Gmail app.
Best wishes, let us know what you find.