How can a Muslim differentiate between the delay in the response to his supplication being due to divine wisdom, or due to his own shortcomings in striving or committing a sin, and what are the sins that may prevent the supplication from being answered?

If a prayer is not answered, the believer should have a good opinion of Allah, for what Allah has decreed is best for them. Perhaps He averted some evil from them or stored up a reward for them in the Hereafter. As stated in the noble Hadith (definition), supplication is answered in one of three ways: by expediting the response, by storing it for the Hereafter, or by averting an equivalent evil.

As for the reasons for delayed response, they may be due to the servant's sins or heedlessness. One should not be impatient for the answer, and should ensure their food and drink are lawful, and supplicate with certainty and presence of heart. The non-response might also be better for the servant, as Allah might avert evils from them that they did not anticipate, or compensate them with something more beneficial for them in the Hereafter.

Along with supplication, one must take measures and strive to seek provision in various fields, for the door of provision is wide.