Hooked, I was in the same position as you just about a month ago. However, I had some decent luck on Saturday and noticed the exact same thing.
Saturday morning, I was using a very yellowish brown Sulphur parachute and had success with it until about 11:30. Note: The entire morning I never really saw a consistent hatch of anything.
Saturday afternoon, I tried to use the exact same set-up I fished that morning. There was a consistent Sulphur hatch that started up and fish were hitting all around me non-stop except they would not touch my fly. I ran into Bigsur and after talking to him, we figured out to go a little bit darker and use something more grey than brown and yellow. That was the trick for me and I was able to catch a few. To me, it was a very small change and almost not enough for me to notice but for the fish, they seemed to notice. I was throwing a 16, FWIW.
I think the density of the hatch coming off and the location you are at on the river can play an important roll. Hope it helps. Each time I go to the river I am learning more and more.