Yes I agree - the rocks may be a tad bit big for them as they look small. Kuhli's, as well as other varieties loaches prefer sand and is probably looking for a hiding place. They tend to hide during the day - if you can find a smooth hollowed out ornament, that would be ideal.
Gravel is larger with smooth edges. Lot of holes in-between gravel...they seam to be able to navigate this substrate well. Have a few larger flat rocks on the bottom the tank. Will move the gravel a little and will act as a couple of caves. Put a few old building in the tank also to give a few hiding spots.