I've seen this type of behavior many times in lakes and haven't solved it. Like you, I empty my midge box when I see all this activity and no bugs hatching.

Did you scan the surface intently and close up? I know the activity is away from shore, but sometimes the bugs wash and accumulate near shore. Could you throw a bug net out there and pull it back?

Pure speculation, but I'll assume you've used all the normal emerger type patterns.

Could it be egg layers? Could be any size. The fish could key on them diving or returning to the surface, so a sinking or rising fly could be the trigger.

Could motion be the trigger? This affects both retrieve and pattern. Since most fly fishers focus on a dead drift, we aren't that good at covering all the little twitches and retrieves that imitate nymphs swimming to the surface. Many times I've hooked fish just after an inadvertent jerk or bad mend.

Could it be aquatic insects, tiny fish, shrimp, scuds, etc..., that are gathering near the surface for some reason? At certain times, plankton and microscopic bugs can rise to the upper water column, and the food chain follows them up. I once saw bunches of snails feeding just under the surface for some reason, and trout were on them. Of course, I had no snail fly on me.

Good luck, sounds very frustrating.