When Indrajit fired Brahmastra towards Hanuman, Hanuman say he has no capacity to liberate himself from Brahmastra. Here is the quote from Sundara Kanda Sarga 48.
"I have no capacity to liberate from the bondage of the missile due to the power of Brahma the father of the world. Thus knowing the bondage, through the missile presided over by Brahma the self-born creator imposed by the enemy, it must be obeyed by me."
But just before Hanuman entered Lanka, Jambavanta reminded him that Brahma gave him a boon of indestructibility from any kind of missile. This is what Jambavanta told to Hanuman as per Kishkinda Kanda Sarga 66.
"Oh, chivalric warrior at war, while the Air-god is being supplicated, oh, dear boy, Brahma gave you a boon decreeing your 'indestructibility by any missile...'
So inspite of having a boon of not getting harmed by any weapons, why Hanuman said he has no capacity to liberate himself from Brahmastra?