You probably need to add a little water to it. Even though you think you don’t have a pond, you do. A liner is placed under the rocks to hold water to operate a waterfall so for future practice you should always see just a small amount of water at the top of the rocks so you know the pond is full and this will not happen. If the pond is still running slow and it is full the bottom of the pump could be clogged and you need to dig the pump out and clean off the filter on the pump, this occurrence is rare. Please call if you think this is the problem, running your pump dry can severly damage your pump. Unplug pump immediately if it is not operating properly.