It is not unusual it is very simple. Even the greatest artist can end up with material that is never going to be a masterpiece and the artist may spend much time on the material before this is realised. Hence it is then the fault of the material not the artist. You have stated that it is always the artist fault if the comosition is a dud.
I guess this happens more often for the less expereinced or less gifted
I am sure Michael Angelo used only the best marble peices but even then he ended up with a dud.
