Adam Clarke on Matthew 17:19

Why could not we cast him out? - They were confounded at their want of success - but not at their want of faith, which was the cause of their miscarriage! When the ministers of the Gospel find their endeavors, with respect to some places or persons, ineffectual, they should come, by private prayer, to Christ, humble themselves before him, and beg to be informed whether some evil in themselves have not been the cause of the unfruitfulness of their labors.