This is often a problem with the impeller. Make sure the magnet and the white blades are connected as one piece. If they are separated, you will need a new impeller. This separation is covered under our 2-year warranty. Another problem that may occur with the impeller is sand. Tetra Whisper® Power Filters will not run properly in an aquarium containing sand. The sand gets between the magnetic impeller assembly and the Energizer®, preventing it from turning. You could possibly replace the sand with gravel or purchase a mesh net from your local pet store. Place the mesh net (or a piece of panty hose) on the strainer. Your filter will still pull out waste but it will not pull up the sand. Gravel is the preferred choice over sand in a freshwater aquarium.