Several quick points... Are you sure the jumpers inside the controller are properly set to cool as opposed to heat?
Is the differential temperature setting correct? 1 or 2 degrees. Is this your first batch using this tank? Did you test before hand to see if the solenoid is wired correctly to close and open properly? Is the solenoid a normally closed valve?
How cold is your glycol? You could possibly have ice build-up preventing the valve from closing.