Louis-
Any ideas, pity, etc. will be welcome.
You are far too good a fisherman for me to pity you, but I will share an idea or two.
How do you know that someone didn't put the big hurt on those fish a couple days before you tried
John's exactly right. Sometimes we fail to consider how recent fishing pressure factors into our daily success or lack thereof. Imagine your best day on a favorite stream. Now imagine the fate of the next poor fellow that follows you on that water. Don't you think he might experience something very similar to what you describe? And it's not like our own success should make us expect that the fishing will be just as easy the next time around--pressure is cumulative, whether we create it or someone else does.
OK, OK, I know there are too many variables here....
Variables--that's the key word. I think that you are grappling with the basic nature of blind nymphing. It's easy to fall into the trap of comparing "nymphing the water" to "fishing the rise" with a dry fly.
When we present a dry to rising fish, many of the variables are removed from the equation. We know the location of the fish. We know it is feeding (at least occasionally) on or near the surface. We often know (or can figure out) what it is eating. And when we present the fly, we can easily evaluate our accuracy and drift. Even when the fish ignores or rejects the fly, it suggests some obvious possibilities about why that may have happened.
But blind nymphing provides no such advantages. Even when we think we have some idea about location, depth, and food, it is hard to be sure. Evaluating accuracy and drift often involves intuition and feel rather than something we can observe directly. (The movement of an indicator is merely a vague and general reference that we can only read consistently in consistent situations.) And determining when fish might be ignoring or rejecting the fly is almost entirely reduced to educated guesswork.
Another negative carryover from dry-fly fishing is the tendency to assume that if five casts to a given spot or drift adequately cover the water (meaning that the fish probably saw the fly) then the same is true for the nymph. But that is not the case except in the very simplest of drifts in very shallow water. Sight-nymphing to fish visibly holding in deep complicated drifts will demonstrate that putting the nymph in front of the fish is a much trickier task than running a dry over that same fish. It may take many times the casts and many variations on that cast before the fly is properly shown to the fish. And with each cast and each variation one runs the risk of letting the fish see the fly in a bad way before it sees it in a good way. Nymphing can be wickedly effective, but, except in certain spots at certain times, it ain't easy!
Finally, my friend, you may simply be expecting to achieve a level of consistency that just isn't possible in some streams. One of my all-time favorite streams is incredibly fickle. I probably know it about as well as anyone alive, but I still wouldn't wager much on my ability to produce a number of fish on any given day. It is gloriously generous at times, and on other (often similar) days it is stingy as hell. These day-to-day differences aren't entirely random, but they aren't entirely controllable either. And some days I just fish BADLY. I have some statistics gleaned from many decades of notes (that I'll share with you privately) that might shed some light on this.
Louis, many fly fishers can only hope to fish as well as you do. Continue to grapple with the variables by all means, but find some small contentment in that. :)
Best,
Lloyd