This is difficult one to diagnose and can be due to over and under watering. The best way to tell if your container is too wet or too dry is to feel the compost about 1 or two inches down from the surface. If it’s bone dry give your plants a good watering. If it’s really wet and even claggy, then stop watering for a while. Yellowing leaves can also be a sign of nutrient deficiency. Try feeding your plants with a multi-purpose liquid feed once a week.