The Bible declares that human beings are given free will so that we can choose to either accept or reject God (“whosoever will, may come”).
Christian doctrine decrees that God can at least sometimes answer prayers.
If a Christian were to pray that a person were to accept God, and that prayer is answered then that person’s free will is violated.
Conclusion: Prayers answered with regards to the actions/decisions of others would mean that this person’s free will is violated. This creates logical problems, and shows that free will and a God that answers prayers are not entirely compatible.