Although I dont consider any bass taken on the fly as average - we do take a lot of fish in particular weight ranges. As much as 80% of the fish we catch are between 1.5 and 4.5 pounds. Bigger fish are exceptional and often are only targeted either in specific weather windows or on particular locations with specific flies. Taking bigger fish without using a 'fly fishing strategy' to do so is a rare occurrence.
There are many days when we wont catch any fish - this is not because the fish are not here but rather because a combination of conditions have forced them to behave in ways that puts them beyond the range of the fly. The biggest barriers to our fly success are suspended particles and debris in the water.