The New John Gill Exposition of the Entire BibleMark 9:28
And when he was come into the house…
the man's house whose child he had healed, or into some other in
these parts, for the sake both of retirement and refreshment:
his disciples asked him privately, why could not we cast him out?
The nine disciples, who were particularly concerned in this matter,
fearing they had lost the power of casting out devils, conferred
upon them, inquired of Jesus, when he was alone, why they could not
cast the demon out of the child, when they had ejected unclean
spirits out of many others; (See Gill on 17:19).