Looks like the sickly plants are in brown pots and the healthy one is in a green pot. I know, sounds crazy and probably is and you probably have some healthy plants in brown pots and sickly ones in green pots, too. Just something I noticed.
If they were doing fine and suddenly went down hill...can you think of anything that has changed? Different fertilizer? Different watering method? New plants brought in? Anything?
How do you monitor the soil moisture levels?
