The plant on the far left (half out of picture) looks very perky. I'm going on the assumption that your water them all equally and at the same time. Have you checked the drainage holes on the bottom of the pot?
I ask this because I had two nearly identical plants who received equal amounts of water and suddenly one of my plants started appearing over watered. It turned out 3 of the 4 drainage holes were blocked by the pebbles I had in the bottom of the pot... Just something to check.