I'd argue that whenever your going for realistic likeness, if theres an issue its a propotional issue. Here, I think in your drawing the jaw is much larger and bulkier than it is in the reference. furthermore, though the details match the reference well (and btw your line work is super clean) I think if it were digital i'd recommend messing around with the liquify tool to see what proportions you could push around for better likeness. Also, consider getting a more high rez image. Its really hard to make proportional decisions with such a small image.