Marlow's key shortcoming is that he is unable to have a conversation with or get to know women of his own class, and is only able to be natural and open with working class women. This is of course the dilemma that prompts the comedy of the play, as Kate realises that the only way she will be able to interest Marlow is if she disguises herself as a serving woman rather than the gentlewoman she actually is. Note how Hastings defines Marlow's problem:
But in the company of women of reputation I never saw such an idiot, such a trembler; you look for all the world as if you wanted an opportunity of stealing out of the room.
As Marlow himself explains, he spent most of his life in either a college or an inn, and therefore has not had the opportunity to learn the confidence that Hastings has with gentlewomen. That has not stopped him from being very confident with serving women, or women he refers to euphemistcally as belonging to "another class, you know." Marlow's central problem therefore is his lack of confidence when speaking to women of his own class.