Is the du`a of a sinner not answered?

Answer to the du`a of one who calls upon Allah is, in principle, an indication of the righteousness and piety of that person, but it does not always indicate that. A sinner’s du`a may be answered as a means of letting him get carried away or for a reason. Allah answered the prayer of the Shaytaan when he said: 

“ ‘O my Lord! Give me then respite till the Day they (the dead) will be resurrected.’

37. Allah said: ‘Then verily, you are of those reprieved’”

[al-Hijr 15:36-37]

This response was not an honour for him, rather it was a humiliation, to increase his sin so that his punishment would be greater – we seek refuge with Allah. 

And it is proven that the Prophet (peace and blessings of Allah be upon him) said: “The prayer of the one who is oppressed is answered even if he is an evildoer, and his evildoing is upon himself.” Narrated by Ahmad. See Saheeh al-Jaami’, 3382. 

Similarly, the fact that a du`a is not answered does not always mean that the one who is praying is corrupt. There are some prayers which Allah did not grant our Prophet Muhammad (peace and blessings of Allah be upon him). He (peace and blessings of Allah be upon him) said: “I asked my Lord for three things, of which He granted me two…” Narrated by Muslim, 2890. 

Allah did not answer some of the prayers of His Prophet (peace and blessings of Allah be upon him) for an important reason, which was to teach mankind that he had no control over anything, rather everything was under the control of Allah. 

See Sharh al-Arba’een li’l-‘Allaamah al-Nawawi, and Kitaab al-Du`a by Shaykh Muhammad al-Hamad.