What are the reasons for the prayer for success in the grade modification case not being answered, and what should be done for the prayer to be answered and hopes to be realized?

Allah knows best what benefits His servant, and His choice is better than the servant's choice for himself. A person might pray for something that is harmful to them. The supplicant will not be deprived of good from their supplication; either Allah will answer it for them, or avert evil from them, or store up reward for them on the Day of Resurrection, as in the Hadith (definition): "No Muslim supplicates with a supplication in which there is no sin or severing of family ties, but Allah gives him one of three things: either He hastens his supplication for him, or He stores it up for him in the Hereafter, or He averts from him an equivalent amount of evil." One must avoid the impediments to the answering of supplications, such as sins and consuming unlawful things, and supplicate with sincerity and certainty that what Allah has chosen is good.