There are a couple of things that might cause the problems described in this discussion, One is whether the Commons resources are properly shared. These Canvas Commons guides include references to the sharing options:
How do I share a course to Commons?
How do I share a resource to Commons?
Note that some sharing options will prohibit other users, even those in the same Canvas instance, from viewing the shared items.
Another problem confronts those institutions that use a vanity URL (like canvas.school.edu). We do at my college and our users cannot access a direct Commons link with that URL, so they have to do the careful search for resources. So when I create resources to share I include keywords in the description to make searching easier.
Another option is to have the Canvas admins at your college identify the resources as "approved," as there is a filter for resources that have been approved for your instance of Canvas. That will make searching much easier. In case you're not familiar with this, here are a couple of Commons Guides documents that describe how (note that non-admin users can be designated as curators for a Canvas Commons instance):
How do I allow and manage approved content in Commons?
How do I manage approved content in Commons?
A final option is to download the resource and share the file in another way (likely host it online if it's too large to email). This workaround avoids the elegance of Commons but still requires the users to import the resource.
I’m going to mark my response as a solution, but there can be more than one. I hope our colleagues here in the Community share other options.