Hi Jeremy,
First of all, please check whether you have configured the proper settings.
1. Check the site's search visibility options at the following location: <your site name>/_layouts/srchvis.aspx
My settings below are for your reference.
2. Check the site search settings the following location: <your site name>/_layouts/enhancedSearch.aspx
My settings below are for your reference.
There are some articles introducing further information about search settings, see
Search does not return all results in SharePoint Online for Office 365 and Enable content to be searchable
In the issue persists, please let us know whether the issue occurs to every site when the users clicked the search iron? If so, it will be nice if you can post screenshots here for further troubleshooting.
Thanks,
Maggie Li