That reminds me of the scene in "Bruce Almighty" Bruce answering prayers - suddenly you understand all prayers just can't be answered. :)
Seriously, imagine yourself driving through a line of streetlights praying to G-d to make them all green. Some of them turn out green and some not. What do you infer from this experience?
My personal resolution of this is that the formula of the probability of G-d answering a specific prayer has an infinite number of variables:
P = "one's piety" x "merit of one's ancestors" x "merits of others (family, town, country)" x "general provision (weather, economy)" x many many more.
Therefore it is true to say that practically, without knowing ALL of G-d's considerations, the chances of any particular prayer (wish) to be answered are 50/50.
Addition: it is true for Tzddikim also, some are answered (our forefathers praying for children) and some were not (see R' Chanina Ben Dosah's poverty).
Related: Illusion_of_control WIKI - sense of taking control over circumstanses.