You may be overwatering them. I honestly don't know what "moisture control" on the label of a bag of potting soil actually means. It could mean that it helps the soil retain water, or it could mean that it drains rapidly. You can tell by looking at the soil itself. Is it kind of sandy/gritty, with a lot of perlite? That would be a rapid-draining soil. Does it contain a lot of largish pieces of organic material, like bark? That could indicate a soil with a lot of peat moss, that would retain a lot of water.
What is the diameter of the container, and how many plants did you put in it?
A good way to tell what's going on would be to gently remove the plant from the container. If done carefully, removing the plant from the container will do it no harm. You will be able to see if the pot is too full of roots, or if the soil is too wet.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams