Imagine the following situation: a brown trout feeding near the surface in front of you. Moreover, it's large (this requires some imagination). You have the perfect imitation. You know that because, during previous hatches of this same species, this pattern worked consistently. With a careful, accurate cast, you make a perfect presentation. Drag-free, it drifts into the trout's window at the right place at the right time. Everything is perfect. It couldn't be better. But... (now you don't have to imagine anything, just remember the many times you've experienced this) it doesn't take your fly. So, what do you do now? You tie on a different fly, and then another and another. You lengthen your leader to see if it's that darn micro-drag. You carefully move into a different position and cast at a different angle. Zilch.
During the two last seasons, I've verified that there is one more parameter that we generally don't take into account or we simply don't pay enough attention to. Consequently, we don't deal with it as something separate from the other two. I'm referring to the trout's degree of wariness in such a critical situation as feeding on the surface. Conditioned by a heap of circumstances, the trout passes through states in which its feeling of security or awareness of vulnerability vary constantly. These states enormously condition the trout's willingness to take your fly, independently of the pattern or the presentation.
Knowing how to identify and understand these various degrees of wariness will allow you to hook a few more fish and, what is more important, to better understand why the trout rejected or didn't even deign to inspect your venerated imitation.
I believe I've been able to distinguish up to four different levels of wariness.
Perfect presentation + good imitation + low wariness = guaranteed take (Or maybe not; after all, this is fly fishing, not mathematics)
The trout's feeling: completely safe and protected.
Behavior: feeding with complete confidence. It doesn't pass up a single fly. Its radius of action does not exceed its safety limits.
Probable situation of the trout: protected by several elements. Bank, vegetation, shade... even directly above. Very near its shelter.
Reaction to your fly: take almost any pattern that imitates the order and approximate size. It may even take an attractor pattern. It might even admit a little drag (levels 2 and 3). The take will be firm and guileless.
Difficulty: the presentation is usually complicated. As almost always, the best cast is no cast at all... just dap the fly onto the water with your rod tip. The thickness of the leader doesn't matter. A thick one is useful because you'll need great control playing the fish to keep it from snagging your leader on a rock or branch or from gaining its shelter
A lower the level of wariness normally means greater difficulty of the presentation but lesser importance of the imitation.
The trout's feeling: barely aware of vulnerability. Doesn't notice your presence.
Behavior: eats almost everything that flows past. Not 100%.
Probable situation of the trout: protected by some element-bank or vegetation.
Reaction to your fly: if the pattern contains a clear, positive-response stimulus, the fish will take it. Any drag at all will spook it.
Difficulty: average. Your drift has to be absolutely drag-free. If your fly is rejected, it's almost bound to be due to micro-drag. It's worth waiting two minutes and lengthening your leader a bit.
Knowing the four levels of wariness that I believe I have identified will allow you to:
1- Gain a mite more understanding of certain apparently incomprehensible rejections.
2- Know how wary a trout is in order to proceed accordingly.
3- Develop the appropriate behavior and tactics for each trout according to this new parameter.
4- Forget fish at level three since they'll be impossible.
5- Know when to wait and how long.
6- Hook a few more fish.