Hello @KatieWright
It sounds like you already know how to view and use it, but just for reference here is the guide: https://community.canvaslms.com/t5/Canvas-Student-iOS-Guide/How-do-I-view-the-To-Do-List-in-the-Stud...
How long are you waiting to check? Generally it should be showing up fairly instantaneously but it could take a few minutes to reflect. Also, are you ever closing the app out completely or just keeping it open? I'd close it. I also suggest clearing the cache on your iOS for the Canvas app:
https://community.canvaslms.com/t5/Canvas-Student-iOS-Guide/What-Student-app-options-can-I-manage-fr...
If none of that appears to fix it, it would be best if you contact Canvas Support and they can take a look at the specifics of your course and assignment and provide better insight.
https://community.canvaslms.com/t5/Canvas-Basics-Guide/How-do-I-contact-Canvas-Support/ta-p/389767
