If Vishnu himself couldn't defeat Ravana with Sudarshan Chakra, how did his avatar (Lord Rama) do that?

If Vishnu himself couldn't defeat Ravana with Sudarshan Chakra, how did his avatar (Lord Rama) do that? Sudharsana chakra failed the previous time because of Ravana,’ s boon. Ravana had a boon that he could not be killed by gods or demons. He won,’t even be a bug in front of Vishnu if the latter choose to disregard his boons. "Ravana said while seeking boon that, 'I shall not be killed by gandharva-s, yaksha-s, or by gods, or by other demons...' and I also said 'so be it...That demon did not express about humans then with his disrespect to them, and evidently, his death does not occur otherwise." So said Brahma to gods. ~ Valmiki Ramayana, Bala Khanda, Sarga 15. And Adikavi Valimki himself told: Whereby, to whom there is an indemnity against death in war at the hand of gods, demons, gandharva-s, devils, birds, or reptiles, excepting humans, and Shuurpanakha saw such an unimperilled Ravana. ~ Valmiki Ramayana, Aranya Khanda, Sarga 32. Because of this reason Sudharsana chakra fired by Lord Vishnu did not kill him but only managed to cause him injuries. He was struck hundreds of times during the wars with gods with the blows of Visnu’s discus as also with the strokes of other weapons in great conflicts. Valmiki Ramayana Aranya Kanda Sarga 32 On the other hand Lord Rama was born in human form to eliminate Ravana so no such boon restriction upon him hence he was able to kill Ravana. O Conqueror of Your foes, Ravana has long practised austerities, by means of which he has won the favour of the world-revered Brahma. That deity has granted him a boon, by which he is rendered invulnerable to all but man. Considering man of no account, he does not fear him. Before giving Sudharsana chakra to Krishna this is what Agni explained about its power.