On the Home Assignments feed on the Web App, only uncompleted assignments are shown by default. You can change the feed view filter to just instead show only completed assignments or All (both completed and uncompleted) to make them easier to find.
In the current mobile app releases, the default view of the Home Assignments feed shows All (both uncompleted and completed), but if you have an earlier version of the app, the default may be to show uncompleted only.
In both the Web and Mobile apps, above the upper right corner of the feed is a Funnel icon. Click this to choose from a selection of filters, including Show All, Show Uncompleted and Show Completed.
If you still can't find a completed assignment, it could be that a Company Admin archived the content. Reach out to your Company Admin, or contact us here on the support site or at firstname.lastname@example.org.