The loud, demanding employee exists everywhere. While occasionally perturbing, at least you know where you stand with them.
Don’t fall into the trap of focusing solely on the squeaky wheel, the quiet employee also presents a particular challenge. Are they quiet because they’re content? Maybe. Or, are they quiet because they’re not engaged?
Of particular concern is the employee that was once vocal and has since gone silent. Unless you’ve fixed their particular problem, err on the side of treating this situation as a silent alarm. Something that must be explored further.
I’ve been the “quiet employee” and can confirm that silence may not be acceptance and contentment, but might actually be acquiescence and resignation. A good manager is aware of this and a great manager will try to address these situations.