If I am not mistaken, the chance for Cooper Ore in Upper Relling is "More Likely". If we assume you are a bit unlucky, you could have failed twice in a row, even with 8x diligence, to dig some ore.
(If you want my advise, try Relling Bridge. It's "Most Likely" there and I don't think there's a place in any safe Tier 2 province with "Certain" chance of Cooper Ore...)