cucumber trouble

I had potted out 4 cucumbers, three in Oasesboxes -self watering pots which consist in a somewhat small vessel for compost (but bigger than that which the cucumbers were in before I potted them on) and a water 'tank' underneath. The fourth is in a 13 (?) inch pot. The Oasesbox ones are all dying.Wilting and turning yellow. Any suggestions? I did wonder if I had overdosed them with iron-based slug bait, but the fourth plant is ok (although it does have more compost) and I am not sure it's that easy to do this anyway.
Could it be too much water? It's annoying as it's a bit late to start growing news ones or indeed to buy new plants.
Could it be too much water? It's annoying as it's a bit late to start growing news ones or indeed to buy new plants.