maybe try a DC pump.
most/all cars use DC motor to crank start engines, and works in sub zero temp, as long as the car battery can still put out the required amps at low temp.
one theory I can think of is the water maybe starts becoming dense, even though not quite freezing, so the pump starting torque is unable to handle the added load. I believe the DC pump controllers will retry with higher torque if initial startup does not spin the impeller.
how big is the ATO reservoir? you can use water heater insulation on the container.
a water heater in the container will definitely solve the problem, though probably not economical/efficient.