No matter how experienced, everyone has an off day where they won't catch a fish. However, if you're consistently not even getting a rise to your fly or a take on a nymph, you're probably doing one or both of these things badly.
1. Presentation. Is the fly drifting naturally? Does it match the speed and direction on the naturals? Has it landed on the water without spooking the fish? That's why, for the most part, fish don't eat bits and pieces of the forest that have fallen in the water. (ignore this last one if you're fishing for bass, they're beyond stupid)
2. Are you fishing in the right place? Not the stream or lake you've chosen but where specifically in that water. call it reading the water as others have said.
Regardless of species, fish are not all that smart, but they have instinct going for and against them. If it even remotely looks like food, chances are there's a few that will try to eat it.