I can't give you specific answers, but past a certain point there are lots of scholars and teachers about whom you would be more likely to find secondhand accounts or references rather than original sources. It's also not uncommon to have only fragments of a person's teachings.

Like Laozi, some of the famous ancient scholars and teachers may have never existed. Like Socrates, the ones who did exist might not have existed in a form we would recognize. Keep in mind that even Shakespeare from less than 600 years ago is mostly a mystery to us, despite being the foundation of English literature and theater.