During midday, most larger trout are often sulking on the bottom of a deep pool. Evenings and into the night is usually the best time to dry fly fish. Trout, especially browns, dislike the sunlight. Trout like easy meals where they will expend the least amount of energy. 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.