“Then came the disciples to Jesus apart, and said, Why could not we cast him out?”
Matthew 17:19 — Greek Interlinear
Church Fathers on Matthew 17:19
The disciples had received from the Lord the power over unclean spirits, and when they could not heal the daemoniac thus brought to them, they seem to have had misgivings lest they had forfeited the grace once given to them; hence their question. And they ask it apart, not out of shame, but because of the unspeakable matter of which they were to ask.
The Apostles had believed, yet their faith was imperfect; while the Lord tarried in the mount, and they abode below with the multitude, their faith had become stagnant.
This is what the Lord says in another place, 'Whatsoever ye shall ask in my name believing, ye shall receive.' [Matt 21:22, John 16:23] Therefore when we receive not, it is not the weakness of Him that gives, but the fault of them that ask.