Had a weak useless electric. Got a new more powerful unit. Better but weak. Walked entire fenceline fixing shorts etc. Much better but not ideal. Put in 3 new galvanised earth rods. Connected them and now red warning light indicating problem. Worse with new earthling ( new earth by itself or with old earth's as well) How can extra earth's cause a problem????? Am baffled
With electric fencing there are lots of tricks and potential problems, so I will try to answer your inquiry, but some of the answers you may already know
Do you have an electric fence volt meter? If not, get one that you poke one terminal into the soil and connect the other to the hot wire.
Do you know what each of your power units are capable of producing? To check this, turn off the power and disconnect the wires from both terminals and then connect the voltmeter terminals to the unit terminals. Turn on the power and check the reading. Some units have adjustable voltage switches, about 2-3000 V for cattle and horses and 5-7000 V for sheep and goats. If your unit only gives 2-3000 V and there is no switch then that unit is only designed for cattle and horses.
The voltmeter is likely to give different readings if the ground is dry or wet, and with how far into the ground that the meter probe is, and the distance that you are from the power unit. If there are no shorts in a long fence, the voltage reading will be higher the further you are away from the power unit.
I don't spray my fencelines, so when the grass is wet I lose more voltage because of the grass conducting some current to earth. So if you tested your old earth in dry weather and your new earth in wet weather and grass can touch the electric hot wire then that might explain your question. Usually some grass touching touching the hot wire does not matter, because when the soil is damp the animal gets a much bigger shock than when the soil is dry. The problems arise in very dry weather when the fence can cause the grass touching it to burn, or when lots of wet grass is touching the wire.
Fences made entirely out of wooden posts and wire do get an induced current set up in the wires near the hot wire. When one of these supposedly cold wires touches a metal gate gudgeon it will electrify the gate, which is a shocking nuisance . This causes some power loss. I get over this problem by driving a steel Y post into the fence line and connect the cold wires to it and use insulators for the hot wires.
When looking for shorts, I check if there is any power in the supposedly cold wires. If one of these has power in it then something on the hot wire is touching that cold wire.
A hot wire that has no shorts will not click. When you can hear the click then you have a short, generally the louder it is the "worse" it is.
To check that the batteries in the meter are working when you have no reading, get the fence to click by pushing the hotwire onto an earthed wire. No click probably means there is a fence problem. A clicking fence shows there is power in the fence so no reading probably means a flat battery in the meter, but might mean a damaged meter.
Does this help? Good luck.