“Prayer busters (causes of unanswered prayer) are prayerlessness, unconfessed sin, unresolved relational conflict, selfishness, uncaring attitudes, and inadequate faith.”

Unanswered Prayers, written by Pat Alger, Larry Bastian, and G. Brooks.
Song lyrics, No Fences (1990)
Context: Sometimes I thank God for unanswered prayers.
Remember when you're talkin' to the man upstairs,
That just because he doesn't answer doesn't mean he don't care.
Some of God's greatest gifts are unanswered prayers.

Source: A Call to Prayer (1867), p. 16

Disputed

Source: Dictionary of Burning Words of Brilliant Writers (1895), p. 221.

Attributed to Teresa by Truman Capote in "An Interview with Truman Capote" by Don Lee Keith, in Contempora (October/November 1970), p. 40, as the source of the title of a work in progress which he intended as a novel, to be called Answered Prayers; no earlier publications of such an attribution has yet been located.
Variants:
There are more tears shed over answered prayers than over unanswered prayers.
Attributed in The Last Word: A Treasury of Women's Quotes (1992) by Carolyn Warner
Disputed

Source: A Call to Prayer

Letter to her boyfriend, Fritz Hartnagel, as translated in At the Heart of the White Rose: Letters and Diaries of Hans and Sophie Scholl (1987), p. 256; edited by Inge Jens, translated by J. Maxwell Brownjohn; also in Voices of the Holocaust : Resistors, Liberation, Understanding (1997) by Lorie Jenkins McElroy
Context: The only remedy for a barren heart is prayer, however poor and inadequate. As I did that night at Blumberg, I'll keep on repeating it for us both: We must pray, and pray for each other, and if you were here, I'd fold hands with you, because we're poor, weak, sinful children. Oh, Fritz, if I can't write anything else just now, it's only because there's a terrible absurdity about a drowning man who, instead of calling for help, launches into a scientific, philosophical, or theological dissertation while the sinister tentacles of the creatures on the seabed are encircling his arms and legs, and the waves are breaking over him. It's only because I'm filled with fear, that and nothing else, and feel an undivided yearning for him who can relieve me of it.

Source: Children of Light and the Children of Darkness