Expanding the Layers Palette

I have an Illustrator file with 25 layers containing thousands of objects. No objects are grouped, I have no clipping masks, and I have no compound objects. My problem is that the Layers Palette does not display the small triangle that is usually visible between the lock and the thumbnail. It is gone for all layers. Using the black cursor tool, I can still select each and every object on screen, and I am free to move them about, edit, delete, etc, but I cannot access them through the Layers palette since no layer can be expanded. This situation does not occur with new files nor other older files. Just this one. These expanding triangles still appear in other apropriate palettes (e.g., "Appearance"). How do I get the Layers palette to display the triangles which expand each layer to reveal their contents?

I am using version 24.3 of Illustrator CC running on an iMac.