I have been having the same problem with my Calathea Rosea and Calathea Ornata. Warning: this thing spreads from plant to plant. This took me a while but I finally figured it out. I saw microscopic white dots on the leaves and examined under the microscope. Please see the attached image (enlarged x80). These show as small white dots on the midrib of the leaves. Now that I know it is some kind of a microscopic insect, I will give the plants a good wash and apply insect killing soap a few times over the next couple of weeks. This probably won't help the original post owner but maybe it will help someone else.
