Some people have a wrong understanding that God is obliged to answer all prayers. And some think prayer (a set of words) brings victory, which is wrong. It is the prayer answering God who brings victory. People come to God because he answers prayers. (Psalms 65:2) The Bible provides some possible reasons why prayers are not answered.
Not abiding:
Those who do not remain or abide in the Lord, and His Word may not receive answers to their prayers. (John 15:7)
Unbelief:
God will not work on those people who are darkened by unbelief. (Matthew 17: 20-21) It is not inadequate faith, but an absolute lack of faith.
Bad relationships:
When a husband and wife are not living in harmony, their prayers are hindered. (I Peter 3:7)
Unconfessed sin:
When the Spirit of God and the Word of God convicts a person, s/he should confess sins and seek forgiveness from God. (James 5:16) Some do not confess instead become stubborn or hard-hearted.
Deceitfulness:
God may ignore the cry, plea, or prayer of a person if the lips are deceitful. (Psalms 17:1) God hates a lying tongue. (Proverbs 6:16-19)
Failure to fast:
Certain things that believers ask require fasting and expression of humility. (Matthew 17:21)
Lack of Bible meditation:
God will ignore the prayers of those who ignore, disregard, and reject the Word of God. (Proverbs 28:9)
Wrong motives:
When people pray with wrong motives and intentions, God will not answer their selfish prayers. (James 4:3) For example, praying for the destruction of some person who is considered an enemy.
Selfish confidence:
Those who trust the vocabulary they use, the length, rhetoric, and flowery words may not receive an answer from God. (Matthew 6:7) Bible-mandated prayer is not a mantra where words, phonetics, and rhythm are important to control gods and goddesses. It is a conversation with the Living God.
Self-righteous:
Those who think they are faithful, holy, doing all righteous things, and fulfilling all rituals will not receive answers from God. (Luke 18:9-14)
Do I evaluate myself when my prayers are not answered?