Why would God not deliver the blessing that an individual asks for as a result of unbelief? I will explain what I think some reasons for causing a blessing to not manifest.
What is unbelief? Unbelief is not believing in God, not believing he will give what is asked of him. It is calling God a liar, it is calling God's word a lie. It is saying that God cannot do it. Unbelief is evil. God abhors evil, so when coming to God with disbelief, you are coming to God with evil. Disbelief is sin.
Believing in God is good. You are accepting God's word as the truth. When a person believes they will receive what they are asking for, they are asking with good in their heart. You are saying that you confess that God is true.
So how can we purge unbelief? We can read God's word in the King James version of the holy bible. We can listen to the witness that other people have given of God's miracle working powers. Looking at God's word for evidence of a lie, is evil. Hearing a preacher preach the word of God. An individual may need to ask Jesus Christ to change their heart into faithfulness, belief, and understanding.
Satan wants to destroy the people on the earth, and one evil device he uses is unbelief. God wants to save the people of the world, and he spreads the gospel of Jesus Christ. Jesus Christ will help you with your unbelief if you ask him. God loves everyone, and desires that they all live and not die.
Matthew 13:58;"And he did not many mighty works there because of their unbelief." John 20:29;"Jesus saith unto him, Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed."
God is able to do all things, even when someone has doubt, so have faith when coming to Jesus for your requests. Believe in Jesus when asking him to be your God, when asking him to forgive your sins.
There other reason is sin. The person praying might be involved in something that God hates. One example is having sex outside of marriage. This can cause God to not manifest a prayer.
All that is needed in this area is to repent and ask Jesus Christ to forgive. Obey God's commandments and ask to be forgiven. If you draw closer to God, God will draw closer to you.
Another problem that can cause God to reject a prayer, is asking for something bad. Be warned though, sometimes he will give it too you even though it's something he doens't want you asking him for. God will also punish you rather than give you want your asking, so do not ask for something that is sinful.