To be honest, controllers rarely fail. Let's start at the top. Work down the following list until the problem is solved. Try using the flow control to partially close the valve. The flow control is usually a handle located in the center of the valve.
Some cheap valves have a flow control screw rather than a handle.
On these valves the solenoid is right in the middle of the valve and the screw is just to the side of the solenoid. If so, leave the flow control in the partially closed position. For a valve to operate it has to have some pressure drop between the inlet and outlet. This pressure drop provides the energy used by the valve to close itself. At low flows some valves don't have enough pressure drop, so you need to create some.