I just got back from 3 days on the South Holston River in TN where the Sulphurs are just getting going. We were able to pick up fish all day long on pheasant tail nymphs and midges and the hatch would get going about an hour before dark providing some great fishing until you could no longer see your fly on the water. The first 2 days a sulphur comparadun was the fly but on the third day they wouldn't touch it. Every fish in the river was jumping out of the water, so I thought emerger. I tied on an emerger pattern I bought from the local shop and still no dice. Finally I tried a pattern that I tied up and that was the fly. I must have caught 15 fish without ever moving. When I rejoined my friends for the walk out they had experienced the same thing but were never able to solve the puzzle. I showed them the frazzled, chewed up fly still attached to my leader and felt pretty satisfied with the whole thing. It made it twice as nice that it was one of my own flies that tricked those selective trout.
Here is the fly that I copied from a BWO emerger pattern (except for the colors) that I bought in a Catskill shop. It rides right in the film but you can see the white wing. It's just a yellowish/orange body with a rust colored tail of z-lon to represent the shuck and a white CDC wing. The natural's wings were really dun colored but the fish didn't mind the white wing.