Hey AB,
You've mastered the most important thing with flies (that I know of anyway)...a SLOW retrieve is paramount.
I often fish from a tube. I tie what I call a "torpedo" bobber onto my main line. This bobber is tapered down on one end. I then use about 4' of leader, and then tie on a leach or woolly bugger (mostly either olive green or black). I also pinch on ONE BB sized split shot onto the leader...just a little MORE than half way to the fly. That helps keep the line from wrapping around itself when casting.
I go slow. Sometimes I stop kicking for a bit. Sometimes I lightly twitch the rod tip or gently tug the line to imitate movement. But, I get a LOT of bites just trolling SLOWLY. It's a lot of fun when they hit a fly near the surface like that.
If the fish are deeper, I remove the bobber and put a split shot or two onto the main line and drop it down. I've done well that way too.
Good luck.