can old culture of s. aureus not ferment mannitol

[thuydeebird]

Is it possible for an old culture of s. Aureus to not be able to ferment mannitol (on an MSA plate) efficiently? The first time that I streak my unknown on an MSA plate, there was growth and fermentation of mannitol. Through other biochemical test, I was able to narrow it down to S. Aureus. However, when I streak the unknown on another MSA plate again, it was not able to ferment mannitol (it remain pink). Is there an explanation for this?