In certain instances in the holy Qur’an, the Almighty Allah has guaranteed to answer the requests of His creatures.
For instance, in chapter Mu’min (Ghafer), verse 60 says:
“And your Lord says: Call upon Me, I will answer you” . (Qur’an, 40:60)
Again, in chapter Hud, verse 3 says:
“And you that ask forgiveness of your Lord, then turn to Him; He will provide you with a goodly provision….” (Qur’an, 11:3)
Another case can be found in Chapter Naml verse 62:
“Or, Who answers the distressed one when he calls upon Him and removes the evil?” (Qur’an, 27:62)
This noble verse says:“The prayer of the distressed one (whosoever he or she may be) is answered by Allah. It is Allah’s promise to a special group. For this very reason, at times of difficulties, the Shias often recite this verse very desperately and plead before Allah to remove their distress (mainly the occultation of Imam). Besides, according to traditions, “Muztarr” (the distressed) refers to Hazrat Mahdi (a.t.f.s.).
Imam Baqir (a.s.) says:
“I swear by Allah that he (Qaem Aale-Muhammad) is the “Muztarr” referred to in Allah’s Book that says, Who answers the distressed one when he calls upon Him and removes the evil.”
Imam Sadiq (a.s.) too said:
“This verse is revealed about our Qaem. By Allah, he will recite two unit prayers at Abraham’s station (Allah’s House) and will pray and call Allah. He (Allah) shall answer his prayers and remove the evil from him and will make him successor on the earth.” 
Respected readers should refer to the book“Naame-Mahboob” (saying No 4, page 87-113) and“Najm-e-Saqeb” (section concerning Hazrat Mahdi’s names and titles).
Thus, the claim that Allah has not promised to positively answer the prayers is disregarded by virtue of the verses of Qur’an.
What was said about the holy Prophet that his request for forgiveness of certain individuals is of no use, it is worthy to note that about the holy Prophet, the following verse has been revealed:
“And had they, when they were unjust to themselves, come to you and asked forgiveness of Allah and the Apostle had (also) asked for forgiveness for them, they would have found Allah Oft-returning (to Mercy), Merciful.” (Qur’an, 4:64)
Besides, it has come down in numerous traditions that requests of parents for their children, and requests of the oppressed in favour of their helpers, etc are answered.
Regarding the verse that says,“O Prophet, whether you ask forgiveness for them or not, Allah will not forgive them” , it refers to the hypocrites who because of their hypocrisy, even the prophet is not allowed to seek forgiveness for them. This only lays stress on Allah’s Wrath upon them and has no relation with the Prophet’s prayers that are answered.
Hazrat Mahdi’s prayers for others would be answered.
The reason he is called“Muztarr” (distressed one) is because of the distress and hardships arising from Ghaibat (occultation) which is also the result of people’s sins. By Divine Wisdom, he is concealed and would reappear one day. Thus, Hazrat himself is supposed to pray for his Faraj. Anyhow, his Reappearance is in Allah’s Hands and whenever He Wills, that particular prayer, too, would be answered as it has come down in traditions that in one of the instances when Hazrat would be standing besides the Ka’ba reciting the verse,“Who answers the distressed one when he calls upon Him and removes the evil?” suddenly, the divine command for his Reappearance would be issued.