Based on my limited freshwater experience, and I emphasize "limited", when I have encountered fish that won't hit I immediately change two things, fly size and then retieve. If I were fishing small nymphs, I'd go up in size. If I were dead drifting, I'd pick up the pace on the retrieve. And when all else fails for me I go to a wooly booger [olive, black, or brown] and work it hard. But then again, I have limited experiense.