One of the reasons of not meeting the needs is committing sins or not having the Serious intention.
Question:
Why aren't some of our needs and demands met despite the prayers?
Answer:
It so happens that one wants his need repeatedly and insistently, but he doesn't want it wholeheartedly. It's obvious that the one, who doesn't try toward his demand, doesn't want it. Like the one who wants martyrdom's position but doesn't try, like the one who wants the knowledge but doesn't study.
Sometimes committing the sin prevent his dua to be met. "O Allah forgive me those sins which hold back supplication!" [1]
And sometimes the need that we want really is not good for us or we do not have the incumbent capacity or meeting our needs misleads us etc.
Note:
[1] Dua'a Kumayl: "اَللّـهُمَّ اغْفِرْ لي الذُّنُوبَ الَّتي تَحْبِسُ الدُّعاءَ"