I wired a 20A GFCI on 12-2 THWN in conduit from my shed sub panel to outside by the well for my water softener. It is not sheltered and instead in a wet rated box with an in-use wet rated cover. Not long after wiring it up it tripped and I could not reset it. So I verified my wiring and replaced the outlet and it worked for a while longer. Went out tonight and found it tripped again and I can't reset it again. Any idea why this could be happening? It is the only receptacle on the circuit and the water softener is the only device plugged in.