Not getting enough water. You need to check the roots to see if they have rotted. Remove the plant from it's pot, carefylly removed the bark from around the roots (soak in warm water for several minutes, that will help make the roots more pliable.) The dark, slimy roots are rotted. The squishy gray roots are also rotted. Trim them off. Repot in new bark. Do not water your dendrobium until the bark mix it is planted in is totally dry. This is just basic general 'rules'. I don't have many dendrobiums, so maybe someone with more of them will chime in with more specific instructions. Good luck!