Download this manual. Around page 5 there is detailed information for installations in areas with poor soil conductivity.
Note that freezing temperatures can greatly degrade soil conductivity. Fences that work well during warm weather may not be all that effective during Winter.
Also, conductivity can be improved by digging a hole about a foot deep and a few inches in diameter (think post hole digger) around each ground rod. Dump in a few handfulls of Copper Sulfate (CuSO4), soak well, then replace the dirt.
CuSO4 can be purchased from many Vets, farm supply stores, (it’s used to treat the hooves of farm animals) and electrical supply houses.