I have a theory involving this. I don't have the time on the Delaware like kray but I've been on a stream or two. First, I'm a firm believer that it's drift over fly in most situations. My father would always tell me it's not the fly, you suck.
Anyway here's my theory, on why we switch flies and all the sudden a fish takes. You come to a pool with a rising fish you make a cast and your fly drags. The fish sees this abnormal movement and will not eat that fly even if it comes through the zone on a natural drift five seconds later. After making a few bad drifts you've learned how to get a good drift by adjusting your position, mend, angle etc. Now you switch flies and make a better drift and wham. Most people walk away thinking it's the fly, I believe they are wrong. I'm not saying this model holds every single time but I've seen it way too many times to be a coincidence.
Very good fly fisherman really make their first cast count.
Your theory is interesting. And there could be a lot of truth to it.
But I still think that changing patterns can make a difference.
During my early years on the D, I had a real problem catching fish during spinner falls.
Traditional poly winged patterns just wouldn't work for me.
One day a was talking to a guy who lived on the river below Lordville, about it.
He seemed like a very knowledgeable fly fisherman.
And he even told me that D trout are pretty tough to catch on spinners.
So, I decided that I needed to come up with a more realistic spinner pattern.
And went to work on it that off season.
I got some materiel called web wing, and made cut spent wings out of that.
Looked a lot more like the naturals to me.
And the following year, finally had success with my spinner patterns.
Or did I - just by coincidence - start making better presentations?