iCloud does only store the last 30 days of photos in My Photo Stream, and only max. 1000 photos. So My Photo Stream may sometimes be empty until you add new photos, and Photos does not show you empty albums, that are created automatically by Photos.
The confusing part may be, that your Mac can keep more photos in My Photo Stream than are actually in iCloud. If your Mac has kept older photos and you reset My Photo Stream by disabling it on your Mac, My Photo Steam may suddenly be empty, because the older photos will not be in iCloud any longer. See: iCloud: Get help using My Photo Stream
And if you enabled iCloud Photo Library, My Photo Stream will not show at all in Photos.