Re: BWO emergers not working during the hatch
It appears the fish were sitting high in the water colum even though the holes were deep. Nymphs can work very well during a hatch but they have to be kept just under the surface rather than fished near the bottom. I have had lots of success trailing them on a short dropper from the back of a dry fly to keep the fly in the fishes window. You can also have luck fishing unweighted flies under and indicator and greasing the first 3/4 of your leader. Swinging a soft hackle on a dry line without weight can be another nice trick for fooling fish during a hatch. The title of your post states you tried some emerger patterns and had little success.
Sometimes during a heavy hatch you will see fish rising and when a dry pattern doesn't work many people will switch to emergers, this works great sometimes but you should also think about the later stage of a mayfly hatch as well. Spinner patterns get overlooked often by anglers during a hatch but some days that is what the fish are keyed in on and we forget about the dead bugs that have already hatched. If you are fishing fish that are below a section of fast water you may also try sinking your spinner patterns as the fish could be eating adults that have been drowned in the fast water and are readily available to the fish. I like to use a small pinch on indicator, Rio's Kahuna indicators or strike putty when I fish spinner patterns. All of these are small inconspicious indicators that can be attached a few feet from your fly and aide you in knowing where your hard to see fly is at.
Good luck with the hatch next time and don't forget about the dead adults that are probably available to the fish.