A good "presentation" of the fly is everything...well almost everything.
I don't know about you, but I learned long ago that when you are fishing to selective trout (dry or nymph), presentation is everything. The absolute perfect fly will almost always be refused by a selective trout if the fly is not perfectly presented to the fish within the fish's "feeding window". If you are fishing in moving water, a good presentation is all the more difficult due to the presence of multiple currents that can affect almost everything (fly line, leader, tippet and the fly. It is the drag that these currents create that is most often the reason your presentation is less than perfect and the trout refused your perfect fly. Selective trout all have an almost magical sense of "normal" or should I say "natural". If your fly moves unnaturally even the slightest bit once it is in that trout's "feeding window", there is a high probability your fly will be refused. Obviously, drag on your fly is a problem but there are several things you can do to reduce it.
1. You can change your position to create a better angle to the fish so that you are casting across fewer of these differing currents. This usually means positioning yourself more above or below the fish. Sometimes you can just slowly move closer to the fish but this carries a higher risk of spooking the fish. Do not assume that if you move more than 90 degrees below the fish that it cannot see you...it can. To become invisible to the fish, you need to be almost directly below the fish.
2. You can use longer, thinner and more supple leaders and tippets to help reduce drag on the fly.
4. You can use "presentation" casts that put more slack line/leader on the water and this can give you a longer "natural" drift to the fish. For more on "presentation" casts click here...
I have mentioned a trout's "feeding window" several times above. This is a cone of "acute" visibility within which a trout looks for food. This cone is approximately 97 degrees. If you are a fly fishing nurd...I guess that includes me, there is a series of long and very interesting and educational videos that all truly serious trout fishers should watch. There is a lot of valuable information about what a trout sees as well as interesting facts about what the fly fisher sees...or thinks they see. Here are the links to these videos. I recommend watching them in order...make a big bowl of popcorn first.
These videos were done by Wendell "Ozzie" Ozefovich