Titus of Bostra

He then seems to accuse the disciples, but his answer is rather a justification of his casting aside his fear, saying, And I besought thy disciples to cast him out: and they could not. As if he said, Think not that I have come lightly unto Thee. Marvellous is Thy greatness! I did not intrude upon Thy presence at once, but went first to Thy disciples. Because they failed to work the cure, I am now compelled to approach Thee. …

Catena Aurea by Aquinas ↗

Bede

And I asked your disciples to cast him out, and they could not. In this saying he secretly accuses the apostles, for the impossibility of healing is sometimes attributed not to the weakness of the healers, but to the faith of those who are to be healed, the Lord saying: "Let it be done to you according to your faith."

On the Gospel of Luke ↗

Bonaventure

And that this suffering was of great duration is apparent from the difficulty of the cure; whence he adds: And I asked your disciples to cast him out, and they could not. The reason for this, however, was that they could not do so by earthly power, according to that passage in Job 41: "There is no power upon the earth," etc.; and at that time the power of divine efficacy was absent on account of human unbelief. Whence Matthew 17: "The disciples said to the Lord: Why could we not cast him out? …

Commentary on Luke, Chapter 9 ↗