I have a question for you...what do you tell those who believe with everything they are and still don't receive it...Biblically Paul is the first to come to mind since I am studying the "thorn in the flesh" at the moment but there are others...just as there are people all over this world that pray believing and never receive...what do you tell them? Is their belief not as "good" as yours?
I'm just curious in what way are you referring to Paul's thorn? Was it a study showing that God doesn't answer all of our prayers?
I don't claim to be an expert on prayer, but what I do know, I know. And I know God answers my prayers. The reason why my belief can be seen as "better" than others is because I am confidant that God hears me (1 John 5:14-15). I know who I am in Christ, and I know my position as a believer, and I am confidant in it, and I expect to receive what I pray for, no matter what it may be (Mark 11:23-24). I can't say for sure why some don't see the answers to their prayers, but I can tell you that when a lot of people pray, they pray in hope, doubt, and unbelief. They throw prayers our in the air hoping that God will take hold of one of them. They are hoping that God is in a good mood that day, and that he will be willing to answer their prayer that day. Others doubt, and they don't even realize it. They ask God for something, and if they don't see an instant manifestation of their their prayers they say within themselves, "looks like it didn't work" and cancel out that prayer. Or they start off in faith, but then worry whether God will come through for them or not which will also stop that prayer if they are not careful. Another thing is that people doubt God's willingness to answer prayers because they feel like they haven't read the Bible enough, or that they haven't gone to church enough, or because of sin in their life, or maybe because they just haven't done enough good deed. That stuff kills your confidence and you begin to feel unworthy of having your prayers answered, which is why you don't see them answered.
God told me something very important when it comes to prayer, and that is, "expectation is the key." You need to approach God in boldness (not arrogance), knowing that he wants nothing more than to answer your prayer (John 14:12-14, John 15:16, John 16:23-24). If you approach him in hope, or doubt, or unbelief, then don't expect your prayers to be answered. Jame makes a very bold statement when it comes to petitioning God something in James 1:5-8, where he boldly states that a man who is double minded, or a person who has faith, but also doubts, should not expect to see their prayer answered . Because if you doubt God's willing to answers your prayers, they won't be answered. If you don't believe God will answer your prayers, they won't be answered.
In all honestly would will tell them to really examine themselves, and be honest with themselves and about their faith (people will say one thing, but in their hearts believe another thing). Ask God for wisdom in the area of prayer. I use to be that way. I would pray a prayer, but then doubt God's willingness to answer it, and guess what, I rarely saw my prayers answered. There were certain areas of my life where I was able to approach God in boldness, but for many others, I had no confidence at all. That's why it's so important to get into the word, and learn about God's promises and what he has already provided. Because how can you doubt in something you already have. If God said it's yours, then how can we doubt it's our then?
I also want to note that just because some people don't see the promises of God come to pass it doesn't mean that they don't happen (this is especially true in the area of healing). People will make doctrines based off of their experiences rather than what God's word says. God makes it very clear that he wants to answer our prayers, but when people don't experience the answers to those prayers they start to come up with excuses as to why God may not answer some prayers. Or write songs like, "sometimes God's greatest gifts are unanswered prayers."
All I'm saying is to be completely honest with yourself, because it sure isn't God's fault.
Hope this helps.
God Bless