I'm probably not going to have the best answer or know why but, in my local stocked trout pond i've had good success on pheasant tails, hairs ears nymphs etc. I've managed to catch only a few on dry flies such as ants/hoppers even though i see fish rising across the pond, I'm not really sure what they eat off the top but its not what im throwing. Tonight was a very good evening for me right before dark i caught 4 on a griffiths gnat, one 18+ brown trout.