I'd argue my best FFing skill is simply knowing what conditions will produce best and where to find the best fishing at any given time.
Y E S. Being at the right place and the right time is the #1 factor. That takes an avoidance of falling into "routine", and the intestinal fortitude to break out of one if it forms. PPL tend to have "their places" and go back over and over short of "terrible" conditions. But the goal isn't to avoid terrible conditions. It's to find great ones. Don't settle for a predictable "meh" day.
But, that said and agreed with, take a group of friends on the same water, at the same time. Certain individuals tend to outfish others. Consistently. Almost every time. I've been the outfisher and the outfished, depending on present company. But it's relatively consistent.
What's that difference? Casting is part of it. I've caught fish that other guys don't bother casting to because it's in a tight place. Or an inch from shore and they know they won't get the drift. I've tried and spooked fish trying too! lol. I've had buddies casting over fish to no avail, and give up, then you walk in and catch them. I've had that happen to me too. Is it fly choice? Proper weighting? Maybe. But often its casting skill. Drag free drift and all that.
But the real difference between those that outfish and those that are outfished is an attitude, I think. Focus and persistence. When things are slow some settle down into a rhythm of casting endlessly. Others move till they find what they want. Some eyes are glued to that rocky bank for hours looking for a beak to break the surface an inch off shore, and others wouldn't see that rise if they were 5 feet away, just not focused enough. Some are too out of shape, or lazy, to cover water fast on a small stream, so instead throw 6 or 7 casts to a pool you only need 1 on. Others look like they are running while fishing.
The secret to becoming a good fisherman is that drive to always want more and better. The willingness to go find it, and not settle for a status quo of "good enough". This includes pre-game of maps, gauges, and all that. The endurance to do it. The willingness to try stuff that seems difficult or frustrating. The persistence to keep trying it even after initial failures.
The rest... casting skill, knowledge, leader formulas, rig setups, proper weighting, fly patterns, experience on large number and type of streams, etc. For someone with the above attitude, they'll figure it out, and pretty quickly. For someone without the right attitude, they'll find success once in a blue moon and fall into a rut trying to repeat it by doing the same thing over and over.