Nymphs can have a difficult time breaking through the surface to hatch. Some will pause and rest just beneath it, recouping their strength. If swirls but no trout noses are seen, or if trout backs and fins but not heads are seen, the fish are probably focusing on these. Some trout may focus on mayfly emergers just as they struggle out of their nymphal shucks. This varies from fish to fish and hatch to hatch.
Fishing pressure affects trout behavior, too.
Many emergent patterns are offered by shops and cataloques these days. Most are designed to be partly on top and partly beneath the water. The dun finally gets out if its shuck and steps up onto the water's surface. A few species of swift-water mayflies leave their nymphal shucks before reaching the surface. These can be imitated by traditional wet flies and soft hackles as well as with more modern designs.