I notice you say you keep the soil moist. That is a common mistake. You want to let the soil dry out before watering the plants. You need to allow the roots to get oxygen and keeping it moist makes it very hard, especially is the soil is high in clay content. To me, the leaves look like a symptom of over watering. The rule of thumb is to let the top of the soil crust over dry and when you stick your finger into it, should be dry to your first knuckle. Could possibly be nute lock out as well. What is the pH of your water going in and the pH of the run off?

GTD