Firstly, your characters need to ring true to themselves. If they're greedy, are you showing them being that in different ways? They need to be characters that could be people we know.
Secondly, your characters should have flaws and virtues and good reasons for acting the way they are. Do they try to hide their faults or are they unremittingly unashamed of them (the I Am What I Am syndrome!). However you portray your characters here, be consistent unless redemption/change is the point of story as it is in A Christmas Carol. You still need to show your character "waking up" to the need to change. One sudden change of heart will not convince readers. Scrooge needed to be visited by all three ghosts to realise the error of his ways after all.
Thirdly, if your characters have different educational standards (and this is highly likely), are you showing the right level of education for the characters? This will show through in how they speak, the kind of vocabulary they use and so on.
Fourthly, can you hear your characters speaking? Do they seem real to you? The first reader you have to convince is you!