Yes, we have all done it, hung around on a hot summers day to last light thinking there will be a Mayfly hatch on our favourite stretch of river.
Waiting! Waiting! and then it's dark and nothings happening. I stand there in the river fumbling around in my backpack for a torch for the walk back to the car. I think what's wrong with the river? I was on this stretch some weeks ago and there was a good evening rise and fish were taking every Mayfly dun coming down the river. So what's happened? Mayfly are tricky. The stimulus to hatch can be because the light is right and/or the water temperature is lower enough.
Ever day the river heats up a few degrees and peaks at 4pm and then cools down over the next few hours until well after dark. Mayfly like the lower temperatures to hatch in.
Is that the sound of a trout slurping down a large Kossie Dun I hear over the hum of the road noise as I drive home? That's cool.
Best Tip: Carry a thermometer and take the water temperature several times throughout the day. If its not dropping - go home.