New Revised Standard Version
Then the disciples came to Jesus privately and said, “Why could we not cast it out?”

Contemporary English Version
Later the disciples went to Jesus in private and asked him, "Why couldn't we force out the demon?"

New American Bible
Then the disciples approached Jesus in private and said, “Why could we not drive it out?”

Douay-Rheims Bible
Jesus said to them: Because of your unbelief. For, amen I say to you, if you have faith as a grain of mustard seed, you shall say to this mountain: Remove from hence hither, and it shall remove: and nothing shall be impossible to you.

Jesus said to them: Because of your unbelief. For, amen I say to you, if you have faith as a grain of mustard seed, you shall say to this mountain: Remove from hence hither, and it shall remove: and nothing shall be impossible to you.

Mark 4:10 And when he was alone, the twelve that were with him asked him the parable.

Mark 9:28 And when he was come into the house, his disciples secretly asked him: Why could not we cast him out?