On the Home Assignments feed, uncompleted assignments are listed first, followed by completed as you scroll further down. You can also filter your feed to just show completed assignments 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.
Above the upper right corner of the feed is a Funnel icon. Click this to choose from 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.