Certified GKH Gardening Expert
It looks like your plants are in a container too large for them. A small container of soil is best for such small plants. Too-large containers are prone to holding onto too much water, which can damage the plants' roots- this may be the cause of your problem.
Try gently digging up each plant with its roots, and repotting the plants in a small container with good drainage. In the new container, check the soil moisture at the surface and water when the surface is dry.