Im gonna take a guess and say that PH might be a problem... As well that soil looks like it has Zero Perlite in it. The roots need some air to breath..

How often do you have to water those plants?

As well do you know what type of soil that is? Or the PH of the water you are feeding them wiith? As well the PH of the soil...

I woudl start with those questions then some peeps will be better able to help ya!!

kd