This is a general comment and does not apply to anyone in particular.
Developing fly fishers naturally assume that all trout in a location are feeding on the same item and if they just could get that magic fly, it would solve a lot of their problems. When I was a Newbie, I fell into this group myself. I thought if I just had someone to tell me what fly to use, I could catch fish.
The fact is that unless the fish are feeding very selectively, many flies will catch it. Not only that, but during non-selective feeding activity, which is most of the time, the fish take many types of food. They are feeding by "sampling the drift". Sampling the drift means that they take all kinds of things into their mouths, and if it is food they eat it, and if it is not food they spit it out.
Sampling the drift is the method by which the trout become selective. If the same food item comes down the conveyor belt on a regular basis, selectivity develops. The fish no longer needs to "sample" to find food. Instead it begins to recognize that item as food and the fish uses a search pattern to find that item. This is how selectivity develops.
Developing fly fishers also assume that if one fly takes a fish in a given location, that fly will take all the fish in that area. That is not true either. Fish populations demonstrate behavioral variance, which is a fancy way of saying they act differently. Even when the fish are feeding non-selectively, some fish will take a certain fly and others will not. At the same time there are some flies that will catch more fish than another fly.
This biological variance helps to explain why, when fishing a 2 fly nymphing system, one fly will catch more trout than another fly, but the second fly still catchs some trout.
So with those facts in mind lets get back to the non-selective feeding. You can see how telling you what fly caught the trout is not a magic bullet in these conditions.
One of the best fly fishers I have ever met is Peggy Harrell
, who guides on the San Juan. She was so good that when she got bored catching fish, she would purposely put on the "wrong fly" to give herself a challenge.
I've seen Peggy fish the crowded Texas Hole, arriving there at 11 AM when all the good spots were taken. She would take the worst fishing spot and begin catching trout. Eventually, the guy next to her could not stand it and he would leave, so then she would move into his place and start to take fish. In one hour she had the best spot because the other fly fishers had left.
What made Peggy so good was experience and the fact that as a woman, she had to be better to get clients. I've fished with her and even when she told tell me what she was doing, she always out fished me.