I stopped casting and studied the rise forms they made as they fed. It appeared that they were feeding on something that was trapped in the surface film of the flow. Perhaps the flies were spinners, the dying form of a may fly after mating. No, that was not the answer.
Several fly changes later I had yet to hook a trout. Ah well, that is why they refer to it as fishing rather than catching.
An evening such as this reminds me of something that Art Flick once told me: “If you were able to always catch every trout you cast to, fly fishing would quickly become a monumental bore.” I guess Art was correct when he said that. However, it would be nice to hook one every once in a while.