What is wrong with this Lemon Orange plant? Its n...

Well, you underwatered that one. it's so weak the top is bent down because it doesn't have the turgidity to hold itself up. Substrate looks very dry, too, so this is not from roots being flooded for long periods of time. probably bad watering habits and just lucky the others are not having issues. always water entire pot. wait for top 1" to dry and repeat. partial watering or superficial watering is a horrible habit to form. If you don't get the whole thing wet, you are doing it wrong, bottom line. simply watering a plant should not cause overwatering symptoms, if it does, you need better drainaige properties in the substrate. Does not look to be the case here, but if soil should be 50% perlite or similar for best rootzone conditions.