Charles Spurgeon Commentary Mark 9:26-29

"And having cried out, and torn him much, he came out: and [the boy] became as one dead; insomuch that the more part said, He is dead. But Jesus took him by the hand, and raised him up; and he arose. And when he was come into the house, his disciples asked him privately, [How is it] that we could not cast it out? And he said unto them, This kind can come out by nothing, save by prayer." — Mark 9:26-29 (ASV)

And the spirit cried, and rent him sore, and came out of him: and he was as one dead; insomuch that many said, He is dead. But 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,—

According to another evangelist, it was from lack of faith.

However, he added:—