The life of faith, always ambiguous in its results ... frustrating at times, since there is no guarantee that prayer, or anything else we might call upon, will consistently "work."
In Mark 6.13, the disciples are hugely "successful" in their efforts to cast our demons and, by oil-anointing, to heal the sick.
What a spiritual high for them it must have been.
Yet, in Mark 9, the disciples have a "mountain-bottom" experience - they can't heal a boy with convulsions. After Jesus heals the child, the disciples inquire as to why they failed. Cryptically, Jesus replies that some "demons" respond only to prayer (NRSV; other texts add fasting), leaving the disciples scratching their heads, I'm quite sure, as this leaves us wondering, too.
But Mark has a point - at no point in time can the disciples "patent" a process of healing ... sometimes it happens, and sometimes it doesn't, and there's no clear explanation of it.
How humiliated the disciples must have been when they failed, and worse, when the scribes went after them. Had the disciples perhaps been earlier boasting, after their earlier success? Heck who doesn't engage in "spiritual boasting" now and again, if not with others, at least within our own spirit?
Mark addresses the simple reality: sometimes it works, and sometimes it doesn't, and there's no pattern to be claimed by the disciples, no method that "guarantees" anything.
Perhaps to keep in check the church's inclination to boast: "my god can beat up on your god," and so on.
Sort of like preaching: that one Sunday when it all came together and the angels sang ... and the preacher goes home to write a book about "successful preaching."
And then the following Sunday, with the preacher in full control of the method, full of confidence and self, and the sermon lands like a cow pie in the field - kerplop! And the preacher goes home humiliated and sad. And later in the afternoon, Jesus stops by and says something about prayer.
Oh well, there's another Sunday coming ... and another chance, and maybe it'll work ... and maybe it won't. Jesus my LORD!