Why a Dua doesn’t get answered?

Sometimes our duas are not answered. Some of those reasons eventually become obvious to us with the passage of time and some don’t. However, as Muslims we should understand the reasons to help us better cope with situations when our Duas are not being answered. This can help us influence factors that are under our direct control.
We know from Quran and Hadith the many reasons behind duas not being accepted. Some of them are the following:

• The Dua that a person is asking may not be good for the person but that person may not know the reasons yet;
• A persons’ sins may be in the way of accepting Duas;
• A person earns his living through unlawful means (haraam means);
• A person may be asking for forbidden or sinful items;

In the Quran, Allah says:
“And it may be that you dislike a thing which is good for you” [Surah Al-Baqarah, 2:216]
According to a hadith of the Prophet (SAW):
“There is no Muslim who calls upon his Lord with a dua in which there is no sin or severing of family ties, but Allah will give him one of three things: Either He will answer his prayer quickly, or He will store (the reward for) it in the Hereafter, or He will divert an equivalent evil away from him.” They said: “We will say more dua.” He said: “Allah’s bounty is greater.” [Narrated by Ahmad, 10749; Al-Tirmidhi, 3573. Classed as Saheeh by Al-Albaani in Mishkaat Al-Masaabeeh, 2199]
On a related note, Umm Salamah said: I heard the Messenger of Allah (SAW) say:
“There is no calamity that befalls one of the Muslims and he responds by saying ‘Innaa Lillaahi wa innaa ilahi raaji’oon, Allahumma ujurni fi museebati w’ukhluf li khayran minha (Truly, to Allah we belong and truly, to Him we shall return; O Allah, reward me in this calamity and compensate me with something better than it),’ but Allah will compensate him with something better than it.” [Narrated by Muslim, 918]
Ibn Al-Qayyim said:

Whatever Allah has decreed for His believing slave is a blessing even if that is in the form of withholding; it is a favor even if that is in the form of a trial, and the calamity decreed by him is fair even if it us painful. [Madaarij Al-Saalikeen, 4/215.]
Many ask if Allah is not being merciful with them when not answering their prayers. (The real reason is that) What Allah chooses for His slave is better for him than what he chooses for himself. Allah is more merciful towards His slaves than they themselves or their mothers are. If something happens to them that they dislike, that is better for them than if it did not happen, so His decree is all kindness and mercy. If the slave submits to Allah and has certain faith that all dominion belongs to Allah and all things are under His command, and that He is more merciful to him than he is himself, then he will find peace of mind regardless of whether his need is met or not.  [Madaarij Al-Saalikeen, 2/215]