I can't answer this, mainly because I think the premise is wrong. It's not that the Ancient Greeks (and Romans) could accurately represent the human form and medieval peoples couldn't. Rather, Classical culture was interested in representing the human form accurately in some media at certain time periods, while medieval art was far less interested in doing so, while at the same time managing to accurately represent the human form in some media at certain time periods. But it all depends on what medium and what the purpose was. Compare classical and Hellenistic sculpture, which was very focused on 'realism' (and, in fact, idealizing the human form) for example, with contemporary vase painting, which is flat, disproportionate and uses distorted space. Similarly, weird, distorted and disproportionate medieval manuscript illuminations were produced at the same time as architectural and funerary sculpture achieved very lifelike (albeit still stylised, and significantly, clothed) forms.

The entire problem with these perspectives really comes from a Renaissance idea that the 'goal' of art is to make a photograph in stone/paint/clay of real life, and that because the Greco-Romans got closer to that medieval artists, medieval art is somehow degenerate, less accomplished or skillful and just generally 'backward'. It's not really a view taken seriously by art historians today.

Out of interest, why do you say later cultures weren't able to? I've seen some Roman busts and statues of famous people like Caeser and they look like believable human figures to me.