The ultimate goal of fly fishing is to imitate a fly or other insect so closely that it fools the trout into thinking that it’s next meal is being served. In order to do this you must “present” the fly in such a way as to not spook the trout. You do this by either “dead drifting” the fly or with some movement to make the fly look like it is swimming or fleeing. No matter which method you’re using, you must make your imitation look like the other flies that are drifting along with it.
Trout position themselves in strategic locations in the stream where the most oxygen and the greatest quantity of food is delivered to them. Trout focus on two major things, self preservation and sustenance. Because of this, trout watch literally thousands of different things float by them and become accustomed to seeing these things float by at a certain speed and taking a certain path, etc. Because of its curiosity, a trout will experiment on occasion by eating items that it doesn’t normally see. This is why attractor flies work. They don’t really imitate anything in nature but you are preying on the trout’s curiosity. If a trout sees any unusual movement, it will either simply ignore it or it will spook it.
This unusual or unnatural movement in your fly is called "drag." Drag makes the artificial fly drift slower, faster or along a different path from real insects and is usually caused by the current pushing or pulling on your fly line and/or leader. Another factor can sometimes be the existence of wind. Drag usually creates a V-shaped wake behind a floating fly. The easiest way to detect drag is to compare your fly's drift with the drift of a living insect next to it. If your fly is not floating at the same rate of speed as an object beside it, you’re experiencing drag and your chance of enticing a trout diminishes greatly.
What can you do?
Mending the Line
"Mending" is a technique for prolonging a fly's drift by adding virtual slack into the fly line. You can mend the line while on the water or in the air during your cast. Mending absorbs the current's push so the water cannot straighten the line too quickly.
To mend the line on the water, as your line begins to straighten out, point the rod tip at the line, then flip your forearm and wrist up-current. This motion is simple but does take some practice to get it right.
To mend the line during your cast after you’ve stopped your forward cast motion, quickly reach your casting arm far to the side. The fly will follow its initial path when you stopped the cast. The fly line, however, will land to the side. This will give the fly more time to drift without drag before the current pulls on the line.
Change your approach
If possible, change your approach and position yourself directly above the trout in the current. By doing this, you eliminate the sideways current between you and your fly. Feed your fly by stripping line off your reel and wiggling the rod tip close to the surface of the water so the line doesn’t get hung up on the guides while feeding out.
If you are casting to a fish feeding at the surface, you can get closer to it than if it were in deeper water. The higher the fish is feeding, the less it can see above the surface. Approach the fish slowly and quietly.
Change your cast
Cast an “S” into your fly line to give your fly more time to float before the current straightens out your line and begins the drag. The moment after you stop your forward cast, wiggle the rod tip from side to side as you lower it to the water. By doing this you will introduce a zigzagging effect into the line thereby increasing the amount of time your fly has on the water without drag setting in.