Usually, during this time of year when there's no major hatch brewing, the case of one fly working while the other tried-and-true pattens fail probably has a lot more to do with luck, presentation, and opportunity than anything else. I bet if you gave an equal chance to pheasant tail nymph or any other proven nymph with a similar size and sink rate, using the same presentation you're using with the hare's ear, you'd catch just as many fish. Since you're contact nymphing, you could experiment with this using a hare's ear as the point fly and something else as a dropper, then switching back and forth every so often, and seeing which fly catches the most fish.
A size 10 hare's ear is a decent approximation to a mature Drunella nymphs, but the nymphs in the river in the moment should be immature and much smaller. You're really encountering fish that aren't seeing a ton of food right now and are feeding opportunistically, but they're also lazy because it's cold, so they mostly won't move very far for food, and a nymph right in front of their nose is the best tactic. If you've tried smaller hare's ears or similar nymphs without luck, my guess is that they weren't sinking down to the fish's level as well as your size 10. I bet a size 14-16 perdigon would catch those fish too.
