I would suggest three things:
1) Make sure you have the latest version of the app. 6.18.0 is being released and should or will soon be available.
2) Reset app cache. On iOS, you can find that in your phone settings: https://community.canvaslms.com/t5/Canvas-Student-iOS-Guide/What-Student-app-options-can-I-manage-fr...
3) Uninstall and reinstall the app.