Jesus Heals When Disciples Couldn't

"And when they had come to the multitude, a man came to Him, kneeling down to Him and saying, 'Lord, have mercy on my son, for he is an epileptic and suffers severely; for he often falls into the fire and often into the water.
So I brought him to Your disciples, but they could not cure him.'
Then Jesus answered and said, 'O faithless and perverse generation,
How long shall I be with you?
How long shall I bear with you?
Bring him here to Me.'
And Jesus rebuked the demon, and it came out of him; and the child was cured from that very hour.
Then the disciples came to Jesus privately and said, 'Why could we not cast it out?'
So Jesus said to them, 'Because of your unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move; and nothing will be impossible for you.
However, this kind does not go out except by prayer and fasting."
Matthew 17:14-21

It takes prayer and fasting to remove unbelief.

Mark's account adds:
"So He asked his father, 'How long has this been happening to him.?'
And he said, 'From childhood.
And often he has thrown him both into the fire and into the water to destroy him.
But if You can do anything, have compassion on us and help us.'
Jesus said to him, 'If you can believe, all things are possible to him who believes.'
Immediately the father of the child cried out and said with tears, "Lord, I believe; help my unbelief!'
When Jesus saw that the people came running together, He rebuked the unclean spirit, saying to it, 'Deaf and dumb spirit, I command you, come out of him and enter him no more!'
The the spirit cried out, convulsed him greatly, and came out of him.
And he became as one dead, so that many said, 'He is dead.'
But Jesus took him by the hand and lifted him up, and he arose.
Mark 9:21-27
Luke 9:37-42

The man brought his son to Jesus.

Jesus taught about releasing faith when He told of the mustard seed.

The man declared his faith and asked Jesus to help him with his unbelief.
He was determined to exercise faith, and refused to act or speak unbelief.

Jesus exercised power and authority when He rebuked the demon, commanding it to come out and stay out.