If yours floats wrong, drifts wrong, or is the wrong size they will usually not take. Try fishing a Rusty spinner when they are taking Hendrickson duns, or try different patterns of the fly being fed upon. Trout become line shy, and can visually see your leader. Try a smaller tippet size. Feed your fly to them accurately, casting over their heads will greatly decrease your odds.
False casts are a deterrent.
Trout become more wary, line shy, and selective the more pressure they get. Try moving up stream or downstream from an area where fishing pressure has been heavy, or venture off to a more remote location. Summary Remember that trout are wary and selective fish. They need to feed like every other living thing, but will only do so when the food supply and needed security is there. Take your time and make each cast count, this is where many people make mistakes.