With unknown loss first thing is check or re-terminate all the wire splices (wire nuts are horrible outdoors, twist and solder if you can, then wire nut with tape protection) and check every receptacle wire for corrosion or other damage to the plastics or screw clamps or plug socket...

Something you also may want to check is if there is any current flowing through the loop when nothing is connected out at the well head.

We had our house and the next door neighbors underground wire runs to outbuildings gradually go bad - with symptoms starting as incandescent bulbs dimming over weeks until they only produced orangery light and then totally open circuit. Since both went bad within a year so nearby lightning caused insulation damage is suspected.

What was happening was shorts were developing that also lowered the carry capacity of that chunk of wire beyond that point. If the conductors are not melted outright over the years the ground moisture produces copper oxide which builds up and spreads a thin layer until current starts flowing in a short circuit that is less than the fuse or breaker rating, which makes heat which accelerates the process until a full arc burns open the circuit or leaves it very high resistance.