"Luck is a residue of skill" - Woody Hayes quoting Amos Alonzo Stagg.

It's how well you understand and apply the three T's of fly fishing - timing, tackle and technique. That's another way of saying what DG said, I think.

For me, what makes fly fishing so interesting and challenging, is all the variables that come into play in just about any outing, and the variables are expanded by a factor of three - the three T's. You can have everything just right in either of two of the T's, and blow it completely on the other one.

And, as I like to remind myself when it happens, and it does often enough to have a saying for it - "You have to do a lot of things right to foul hook a whitefish."