Hey there Sadies-mom,
Thanks for reaching out to the Apple Support Communities. We understand you're having issues with contacts not showing up for Yahoo mail, and we'd be happy to get you pointed in the right direction.
To get started, when you say that you're using Yahoo mail, are you referring to the Yahoo Mail app, or are you using an Yahoo account with the native Mail app on your iPad? If using the Yahoo Mail app, give the steps listed in the following resource a try that may be able to help: If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support
While we understand you're not having issues with the app quitting or not responding, the steps above can help with troubleshooting other issues with third-party apps as well.
If still having issues after going through those steps, the next best option would be to reach out to the app developers who may be able to provide you some additional information or troubleshooting steps. For help with reaching out to the app developers, check out this next link here: How to contact an app developer - Apple Support
We hope that's able to help.
Take care!