Every kind of prayer, not intercessory prayer only, which is the highest kind of prayer, but all prayer, from the lowest kind to the highest, is impossible in a life of known and allowed sin.

Alexander Whyte (Preacher, 1836 – 1921)

What he is saying, is that our sin interferes with our prayer life according to Scripture. God will only hear the prayer of repentance, before He starts answering any of our other prayers. For we need to be right with God in order to have our prayers answered. They are all heard, but not necessarily answered in the affirmative if we are in sin.