They are visual hunters, they like to see their food moving to trigger the feeding response. See if you can find some live brine shrimp and soak them in a little selcon or phyto to gut load them(more nutritious for mandarin). Mine also eat live black worms.
I had problems with other fish eating them too quick, the solution was a gatorade bottle with the top cut off that had a tube coming to the surface. I shot food down the tube and the mandarin would pig out. It helped to keep all the food in one spot, in the tank it would float away before they could eat it, even with the circ pumps off.