Some birds are more tuned in to singing rather than talking. Many times a bird that picks up on whistles and songs stays with that sort of mimicry instead of actually talking, but there are exceptions.
A bird usually learns to talk for a few reasons. One, for the attention the human gives them when they do speak. Two, to communicate with their beloved human. Three, to fit in with the human flock.
What helps is that the bird is really bonded to you and therefore wants to "speak your language" and be part of that "human flock". Learning the language of their flock is one way to fit in. Another thing that helps is if the bird can associate the word with an action or an object...such as "bath time" or "good morning" or "carrot". That is how birds will learn to communicate and say the right thing at the right time, letting you know what they want and when, or just expressing their personality. That reminds me of a story...anyhow... Your bird might also learn to talk for the attention he receives because he notices every time he says "Hi" (for example), his human flock members run to his cage smiling and spending time there with him.
Singly kept pet birds are often better talkers than birds who have a "bird flock" at home too. If the bird has other birds to communicate with and speak "bird language" with, they may not be so inclined to try learning human speech. Also birds that were not properly socialized and who may not want a whole lot of attention from their human flock, those birds may also not be interested in learning human speech. Birds who are sick, bored, stressed or frightened might also not learn to speak. Just like people - when you're happy and feeling good, you like to talk and when you're not feeling well, you don't.
Start with simple, one- or two-syllable words such as "Hi", "Hello", "Bye", "Night-night", "Yum" - and don't forget to lavishly praise your bird when he does say what you want him to.
Just because your bird doesn't ever learn to talk, that doesn't mean he doesn't love you or want to be part of your flock.