why does Teiresias initially refrain from divulging the truth to Oedipus? What causes Teiresias to change his mind and speak the truth?

He does not think Oedipus will believe him and sometimes, as a prophet, keeps things to himself.  Eventually Oedipus accuses him of being in league with "bribers" and so Teiresias tells him the truth which Oedipus does not believe.