Our best advise during the mayfly hatches is ~ A Good Drift is Better than a Good Cast ~ After mayflies hatch they drift on the surface like tiny sailboats until they take to wing. They seldom bounce around on the surface as do caddis when they hatch. The bottom line is, if your mayfly drags at all fish will probably not even consider eating it.
Also when you spot a feeding fish don’t be in a big rush to drift your fly over it. Instead be patient and watch how the fish feeds. If you time the intervals between rises you’ll find it may be a few minutes or more. After a fish rises it has to return to the bottom, get back to that sweet spot in the current and begin looking up for another treat. Drift your fly over the fish too soon and it’s not even ready to feed again. The fish is going nowhere so you have plenty of time to make that perfect drag free drift over the spot where the fish is taking bugs. A little patients goes a long way especially when it comes to big fish.
Fly choice is simpler than you might think. Once you find a feeding fish don’t be in a rush, again the fish are going nowhere. Look for bugs on the surface. Follow them as they drift over feeding fish. Spot one eating a particular bug then have a look in your fly box. Make such to choose the same size and color fly. After you tie it on cast it close to the real thing for a comparison, if it’s bigger or smaller it likely won’t work. Make the adjustment and your odds of getting a feeding fish to take are way better. Don’t hound a fish with a fly it doesn’t want, you can’t make him want it. Three or four drifts will tell the story.
If at first you don’t succeed try another fly.