Spinners and emergers present an exceptionally easy meal for them. Trout are often very selective and fussy. This is probably why most people enjoy trout fishing, they just don't realize it. Leaving a river frustrated makes me want to fish it harder the next time. There is a satisfaction to finally see a trout take your dry fly, seeing your line twitch from a trout striking your nymph, or the jolt on the end of your line as you strip your streamer.
Trout are especially selective on rivers with a population of wild fish.
A trout that has been feeding on a certain insect, knows exactly what it is saposed to look like. Try to time the fish as it takes. Count in between when the fish takes, and try to float your fly over him when he's ready to take again. Try throwing a reach mend into your cast, or mending to keep the fly drifting naturall as long as you can with fishing with a quartering downstream approach. Trout will usually select a certain insect to feed upon, and consistently take it even if there are six different mayflies on the water.