Can a person go to the Clinch and catch a fish at 8 am on a sulfur dry right now? Absolutely, they are aware the bugs are hatching and are keyed into their presence, so they will take one if presented properly.
However, some of the fish won't until the nymphs become active, and adults begin to show up on the surface.
This goes back to the perpetual view that trout are stupid. In other words, some if not most can't remember what happen 12 hours ago, and it takes a good number of bugs to get them in the groove of eating them every day.
Thank god for the dumb ones with short memories, if not it might be impossible to catch more than 1-2 a trip.