“The most common cause of unanswered prayer is prayerlessness.”

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.

Disputed

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

In Search of the Miraculous (1949)
Context: When we speak of prayer or of the results of prayer we always imply only one kind of prayer — petition, or we think that petition can be united with all other kinds of prayers.… Most prayers have nothing in common with petitions. I speak of ancient prayers; many of them are much older than Christianity. These prayers are, so to speak, recapitulations; by repeating them aloud or to himself a man endeavors to experience what is in them, their whole content, with his mind and his feeling.

Too Busy Not to Pray (2008, InterVarsity Press)

Source: The Lion

Reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 444.

Source: The Purpose Driven Life: What on Earth Am I Here for?

Too Busy Not to Pray (2008, InterVarsity Press)