What about humidity? You might try enclosing one in a large plastic bag (mini-greenhouse style) for a few days and maybe put a tray of pebbles or perlite inside the bag with water to create a humid environment. It might help the leaves to re-hydrate and get going again. There again, if it is a fungal situation it will make matters worse. But at least it will provide some info.
If it works, I'd gather all the plants in one area and see if you can keep them humidified for a while. Hanging wet towels in a small enclosed room can boost humidity really well for a short time. When you get the windows fogging up, you know you're winning.
Have you tipped one out of its pot to check out the roots? Look at them with a magnifying glass if you can. That might shed some light on the problems, too.
