Guys: I have spent a great deal of time observing feeding trout, both by media and direct observation. I'd like to make a few comments for you to with what you please...

1) With the exception of an oddball rooting fish or a trout that takes a crayfish--I very rarely see trout feed down. Almost all feeding is done laterally or above.
2) foodstuff if the drift does not track in a straight, direct route downstream. It is bouncing, dipping, stopping, reversing direction. Why is this important? I strongly feel that inappropriate use of weight or tippet, be it too much or too stiff/short, acts to "harden" a presentation. Also, NEVER be afraid to WORK YOUR FLIES! If you spend time watching top-level anglers, they will commonly move their flies.
3) Trout take in just about anything that even remotely resembles food when they are actively feeding (seeking food) and will react to triggers when not actively feeding (in resting lies). The crux is that they simply expel much of what they discover to be non-food very quickly.
4) I am convinced that aside from just outright terrible presentation, most anglers fishing subsurface get far more takes than they ever realize--do in part or in full to a blend of being out of touch with their flies or simply loss of focus due to lack of confidence.
5) Too many anglers never learn how complex a stream environment is with regards to hydraulics or fluid dynamics. Because so, much of the very productive water never gets fished correctly--or at all. Understand that the current slows, and often reverses, as you get closer to the stream bottom.
6) Strike detection is the proverbial pot of gold we are all seeking to perfect. In many cases a floating indicator is a nemesis...in some cases it is a godsend.