Fish are also taking those olive leech patterns as Damsel nymphs. Damsels have paddle shaped tails, and resemble a small baitfish or fry in both color and profile. They are also readily available to trout in nutrient rich ponds and lakes that have plenty of vegetation.
John Barr is a famous Colorado tyer that has created a number of productive fly patterns including the Copper John so it shouldn't surprise anyone that his stillwater bugs catch fish as well. His Damsel Nymph is a very clever representation of the insect that everyone should have in their boxes. Fish it under an indicator as part of a two fly system, or by itself with short strip retrieves.