Triggering devices, Fly design and Selectivity
Every fly imitation has four triggering devices: Size, Shape, Color and Behavior
I believe a trout notices the primary trigger, approaches the imitation: and if there is nothing wrong with the secondary triggers will take the imitation.
For example, a trout sees the silhouette of your trico imitation, approaches, then sees over dressed, un natural bulky wings, and refuses the fly.
The angler needs to evaluate the natural then make sure the fly he buys or ties captures the primary trigger. The secondary triggering devices should also be present, but I would rather skip the secondary triggers than capture them at the risk of hiding the primary trigger or making a mistake. Selectively feeding trout are looking for what is right not what’s wrong!
If you’re getting refusals, (assuming your drift and tippet sizes are correct), make adjustments to your fly. Legendary Pennsylvania fly fisherman, Charlie Fox called it a game of nods. If a fish would refuse his pattern, he would start trimming it down his fly. He would continue to adjust his fly until the fish took it.