One of my orchids seems to be poorly. As you can see from the attached photos, the leaves are droopy and most of the leaves have prominent ribs (or perhaps the flesh between the ribs is thin). I have two other orchids of the same species, but they are fine. All of them are on (separate) windowsills. My knowledge of orchids is very basic and I have no idea what the problem might be – under- or over-watering? disease? pests? I'd be very grateful for any advice.