Hi there, I don't know if I understand correctly your situation: if your transformer primary (input) is 440 V, and your transformer secondary is 20,000 V, then, you must be referring to CURRENT in amperes, not voltage...
Then, if one transformer (or more correctly) one field of the electrostatic grid has its current varying, it is most probable that you are handling a crude type that has high conductivity, or that some zones inside the dehydrator have conductive emulsion accumulating, that is draining the electric charge from the electrodes.
Another possibility, is that the electrical insulators in the circuit of the affected transformer are failing and need to be replaced. conductive emulsion can "bridge" those insulators, sometimes called "bushings" and affect their insulation, so that the voltage colapses and the amperage raises above the desired value.
Amclaussen, Mexico City.