Most trout feeding is below surface where they forge on Nymphs. Insects drop their eggs on the surface and these drift to the bottom of streams and rivers where they stay until hatch and the newly developed nymphs are prime food for hungry trout. Their are hundreds of nymph patterns available with Prince nymphs, Hare's Ear nymphs and Pheasant Tail nymphs being the most popular nymph patterns sold.
If you follow the life cycle of a fly there are 3 usual phases of flies; eggs, nymphs and then the flies whose life cycle may be as little as 1 day!. Nymphs here represent insects in their sub-surface and emerger stages of aquatic life. This stage comes before the adult stage where the insects emerge out of the water and fly away. The final stage is the dry fly where the fly mates and lay eggs and the cycle repeats itsefl. The term 'Nymph' it is commonly used to refer to any insect in it's aquatic life stage. Nymphs are, perhaps one of the most deadliest ways of taking trout because most trout feed sub-surface. Sometimes nymphs are weighted in order for them to achieve the proper depth. This additional weight makes them a little harder to cast but the good news is that there is almost no wind resistance. Generally fish nymph flies along the bottom, move them slowly and smoothly. Every now and then dart the fly forward as if it is attacking its prey or trying to escape from the advances of a predatory large fish. Such movements hopefully may induce a following trout to take your fly.
Duffer's fortnight is dreamed about by fishermen when during a hatch or during a spinner fall we watching our fly being gently sipped under the surface of the water by a large trout. However; reality is that 90% of the time when there is not sufrace action we must use nymphs and fish sub-surface. The trout are still feeding, we simply cannot see it! You can keep casting away at likely hot spots with dry flies but you would have more success if you placed your fly where the fish were feeding and that is under water.