Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
The believer in this world is tested and tried, and hardships, limited provision, and failure may be means of raising one's rank and expiating sins if a person is patient and seeks reward from Allah.
Allah, Exalted is He, has established universal causes for outcomes, such as studying, learning, working, and gaining experience. These are shared by Muslims and non-Muslims alike, but they do not independently produce results; all causes and effects are in the hands of Allah. If He wills, He allows them to produce their effects, and if He wills, He prevents them from doing so, for a wisdom known to Him.
As for religious causes, they are things that Allah has made causes according to the texts of Islamic law, such as piety, supplication, and maintaining family ties as a means of increasing provision. It is not correct for a person to abandon the universal cause and rely only on acts of worship while waiting for the desired result. For example, seeking success in one's studies requires studying and acquiring knowledge, while also seeking Allah's help and engaging in supplication and obedience.
Therefore, failure does not necessarily mean that it is caused by sins, nor does it necessarily mean that it is caused solely by negligence in taking the universal causes. A person may take the necessary means and still not succeed, because Allah may prevent some causes from producing their effects as a test, to ward off some harm, to preserve a greater good for the Hereafter, or for a wisdom unknown to the servant. Likewise, it is not correct to regard wealth, employment, or worldly success as evidence that Allah is pleased with a person. Allah gives worldly things to those He loves and to those He does not love. As for true virtue and happiness, they are attained through faith and obedience.
Therefore, the Muslim must take the universal causes, improve his relationship with his Lord and with people, remain patient with whatever befalls him, and not interpret every delay or failure as a punishment or anger from Allah. A hardship may sometimes be caused by a sin, and it may sometimes be a test or have another wisdom behind it. Allah knows best the true reality of the matter.
True misery is the misery of the heart caused by turning away from Allah, while true happiness is the happiness of faith and trust in Allah. Allah, Exalted is He, says:
“Whoever does righteous deeds, whether male or female, while being a believer, We will surely grant them a good life.” [An-Nahl: 97]
And Allah knows best.