Mark 9:14-29
And when He came to his disciples, He saw a great multitude about them, and the scribes questioning with them.....
16 And he asked the scribes, What question ye with them?
And one of the multitude answered and said, Master, I have brought unto thee my son, which hath a dumb spirit;
.. he teareth him: and he foameth, and gnasheth with his teeth, and pineth away: and I spake to thy disciples that they should cast him out; and they could not.
He answereth him, and saith, O faithless generation, how long shall I be with you? how long shall I suffer you? bring him unto Me. ...
....have compassion on us, and help us.
Jesus said unto him, If thou canst believe, all things are possible to him that believeth. And straightway the father of the child cried out, and said with tears, Lord, I believe; help thou mine unbelief.
Jesus rebuked the foul spirit, saying unto him, Thou dumb and deaf spirit, I charge thee, come out of him, and enter no more into him....
Jesus took him by the hand, and lifted him up; and he arose.
And when He was come into the house, His disciples asked him privately, Why could not we cast him out?
And He said unto them, This kind can come forth by nothing, but by prayer and fasting.
Jesus was teaching them that all demons are not all the same level or strength as each other.
This one obviously needed more than but a simple expelling...it needed extra prayer and fasting also.