It's not that he can't mark them well.
Purely confidence to do it under pressure of game scenario where innately just wanting to beat his man/kill the contest.
As woosha said. Going back to twos where he can have pressure taken off and feel more comfortable flying for marks/ leaving his man to go intercept with less on the line - 4pts VFL vs 4pts AFL
Hartley ultimately is still just learning his craft and not settled as a walk up best 22 defender so understandable he'd feel nervous about stuffing up. They need him to just back himself regardless.
If he gets that right and it takes time, Hooker was nowhere as a defender at Hartleys age, then he has everything else to be a topline defender.