Every prayer we pray is opposed by spiritual forces arrayed to prevent the prayer from producing results. These forces must be overcome to receive from God.
While a captive in the kingdom of Persia, Daniel prayed for understanding. His prayer was opposed.
I (Daniel) lifted up my eyes, and looked, and behold a certain man clothed in linen, whose loins were girded with fine gold of Uphaz, his body was like the beryl, and his face as the appearance of lightning, and his eyes as lamps of fire, and his arms and his feet like in color to polished brass, and the voice of his words like the voice of a multitude. … And he said to me, O Daniel, a man greatly beloved, understand the words that I speak to you … from the first day that you set your heart to understand, and to chasten yourself before God, your words were heard, and I was sent for your words. But the prince of the kingdom of Persia withstood me for one and twenty days: but lo, Michael, one of the chief princes came to help me. … Now I am come to make you understand … (Daniel 10:5-14)
Fortunately, Daniel had faith and patience. He continued in prayer until he received the results of understanding.
Paul told the Thessalonians that Satan had hindered his efforts to visit them and to the Ephesians he wrote:
Our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. (Ephesians 6:12)
To succeed in prayer we must know that our prayer involves a battle against spiritual forces that actively oppose God and us. When we ask God for something he does not withhold from us: he gives us what we ask and he does so without condition. However, we live in one of the kingdoms of this world and the kingdoms of this world are currently ruled by forces opposed to God. Those forces do not want us to receive from God. They don’t want any of God’s will to be done in the earth: nothing that pleases him or brings glory and honor to him. They don’t want the results of prayer to be produced and will do everything they can to stop them. To receive results in prayer we must overcome the opposing spiritual forces. Knowing that the opposing forces are withholding results in prayer from us and that God isn’t is one key in overcoming them.
Another key is understanding that we do not wrestle against ourselves. Satan loves to misdirect our efforts: to have us try to wrestle against God to change his mind, or wrestle against ourselves. It is easy to blame God for not answering prayer, or blame ourselves for the prayer’s failure. Not enough faith, doubting, asking for the wrong reasons, etc. are valid reasons for failing to get results in prayer - but why are these a problem? What is their origin? They are the result of seed sown into our heart by Satan: the result of temptation that has conceived to bring forth fruit that hinders our prayer. Overcoming these hindrances to prayer requires that we wrestle against the source and operation of these sources, not against ourselves.
A third key is understanding how the spiritual forces work to oppose our prayer. Prayer works by and is governed by spiritual law. The spiritual forces opposing our prayer do so by spiritual law and work to expose legal reasons why we should not receive the result. God, on the other hand, works to show legal reasons - which include heavy contributions of mercy and grace on his part - why we should receive the result. Jesus is our Advocate, Intercessor, and Mediator.
As time passes, we have more and more opportunity to learn to pray as we ought: to learn what faith and doubt are, etc.. If we don’t learn and our prayer doesn’t improve, then we are likely to quit receiving. God cannot be righteous and overlook problems in our prayer that are only there because we haven’t grown in prayer as we should have. If we fail to grow in prayer we give the opposing forces the legal right to withhold our receiving.
Putting on the armor of God is another key. If Satan's legal challenge fails, he may attempt to cause us to give up on our prayer by attacking us with fear, doubt, persecution, etc.. The armor of God is essential to being able to withstand his attack and receive from God.
The final key is authority. When Satan tempted Jesus he showed Jesus all the kingdoms of the world and claimed to have authority over them - it had been turned over to him and he could give it to whoever he wanted to. (Luke 4:5-6) Jesus did not dispute his claim. We live in territory ruled by enemy forces. Sometimes, even when we have every legal right to receive, the enemy works to withhold our receiving. Not only must our prayer be lawful, but we must have authority to receive and we must exercise that authority or we won’t receive.
Fortunately, Jesus overcame Satan and now has all authority in heaven and earth. Even more fortunately for us, Jesus gives us authority to receive. In the name of Jesus (in the authority he gives us to legally represent him in the earth), we have authority over all the authority of the enemy. What must we do to receive this authority he give us? We must receive it by faith and we must be submitted to him. According to spiritual law governing such things, a person receiving authority from someone must be submitted to them in order to lawfully receive it.
Submit yourselves therefore to God. Resist the devil and he will flee from you. (James 4:7)
Satan won’t flee from you if you don’t have authority over him and you won’t have authority unless you are submitted to God.