You might have air in your system. Start and stop the pump a few times to vent it. Check that the bottom valve is correctly set. Check that the pump is installed as described earlier. Check for blockages in dip tube or elsewhere.
Check that the cable is correctly installed and that the pump speed is set to max on the control system. If nothing happens, turn of the pumps, disconnect the cable and remove the pump. Check that no particles is blocking the impeller or that sugars have hardened inside, locking the impeller.
Remember that the wort contains sugar. It is important to circulate using the pump as part of the cleaning process to ensure the suger is washed out and not hardens as it can cause the pump impeller to get stuck.