LetortRun
If you are unable to interest rising fish in taking your fly and you are being very stealthy and not allowing too much commotion when you cast, I would guess wrong fly. Try a 20 to 24 Griffis Gnat. If they won't pay any attention to that then they may be sipping them before they reach the surface. Try a #18 or #20 Snipe and Purple tied on either standard or, preferably 1x short shank hooks. The bodies should be thin and extend from a point directly over the barb of the hook forward. The hackle should be from the underside of the snipe's wing about where the bird's wrist is. Starling has been a standard substitute but in my experience the Starling feathers are weak and break too easily. The tying thread should be purple and can serve as the body thread. Silk or rayon can also be used. If the little black stone flys are on, Same fly size 14 or 16 would be a good try. You will not be able to see the fly since it will be submerged but only a very little. You may see the hit but don't try to "set" the hook. You will be using a 7x tippet. Some guys use 8x but those who use it successfuly (few breakoffs)
are the experts of our sport. You ain't there yet and neither am I...and I've been at it for sixty years.

Try it, I think that it will work!

Ol' Bill