As others have already pointed out, you should first work on your characters' personalities and peculiarities before trying to write entire chapters with their voice. How do they talk? Do they have a habit of using conjunctions more than others? Does one of them swear more than the others? If one is of a higher class, they should use fancier speech and more educated (but not necessarily more intelligent) phrasing and wording. If another is from a bad side of town, they should sound down to earth, either friendly and welcoming or hostile and suspicious of others, whichever type of character they have.
How do they see things? A more philosophically- or literary-minded person would note (seemingly at random and with a musing, absent quality) allusions referenced, or irony, hyperbole, symbolism, perhaps suspect a bit of foreshadowing at points. Someone who looks only at the present would simply tell what had happened, and their different slants on the world would dictate descriptions of people, environment, and settings. An anxious person would worry excessively and wonder at everyone's hidden meanings. Take all these things to heart, and it'll be easier to create different voices for your characters.
I hope this helps you, and good luck writing!