A facet that's set as searchable and appears to return "No Results", can be caused by having more than 65 searchable facets and searchable attributes combined. This is a current limitation of the engine.
Any search performed on the >65th facet attribute will result in an empty list of facet values, i.e. "No Results".
You can read more on the search for facet values method here.
This facet-searching limitation only affects facet values, not the index search.