Sonny,
That's a good question! ........... :dizzy:
I just bought and read most of Rick Hafele's book/dvd: Nymph-Fishing, Rivers and streams.
I would generally say....... you have to fish the fly-pattern at the depth you expect the natural insect to be at.
You may be fishing your pattern at the right depth, but the fish are not feeding on that insect............ that's a possibility!
To be sucessful ( according to Rick), you have to determine what the fish are feeding on( where you are presently fishing), and fish that pattern at the proper depth.
That is what I learned from the book/dvd.
*Edit...... if you assume all fish are feeding on the bottom, you would assume wrong!
Qg.