Hmmmm...the best way to describe it is that their thoughts and feelings are not pure or refined enough to produce Song Magic. Humans feel and think so many things at once, so they cannot properly concentrate for Song Magic due to those fluctuations. [That may not be the best explanation, so she tries again:]

Think of it like this-- the word 'chs' means 'to change'. A human may be thinking they want to perform the word, change, but other feelings...say, bodily sensations such as hunger, would also be added to the thought 'to change'. Thus, the song cannot be fulfilled because there are other thoughts getting in the way.