Why can't I see the documents in a document set?

I have an existing document library with content types, to which I have added the generic "Document Set" content type. I can create new document sets and upload docs to the document sets, but I can't see the documents on the document set page. It says "Drag files here to upload", but I would expect to see the files that I have already uploaded. If I create a view on the document library with setting "Show all items without folders", I can see the files, and I can also see them under the document set folder when viewing with Explorer. If I create a brand new document library and add Document Set content type, everything works fine and I can see the files that I upload. It's just this one Document Library where I am having a problem. Is this an issue of settings that I could resolve, and does anyone know what settings would affect this, or should I approach this as a bug? Of course, I would really like to avoid re-creating the library from scratch.