Over the past several months, for about a year to be more exact, I have been learning about efficacious and non-efficacious prayer.
For each person, there is what I would call an "authority" for certain prayers. This authority has to do with two things: one's vocation and the Will of God.
One's vocation leads to an intimacy with God which means that the prayers said in connection with that vocation are most likely to be answered.
The obvious example would be a mother or father's prayers for their children; or, a husband or wife's prayers for their mate.
This authority in prayer mirrors the authority one has in life in one's vocation. For example, when I was teaching and had authority over students, my prayers were more efficacious than I realized.
As a mother, my prayers for my son bear fruit, and so on.
When nuns and monks are called by virtue of their vocation to pray for others' intentions in intercessory prayer, especially for priests and leaders of the Church, their prayers are efficacious.
And, these prayers are made in sanctifying grace, of course, and even more efficacious if made without egotism, after one is purified in the Dark Night.
I have discovered which prayers of mine are more efficacious than others.
Secondly, God manifests His Will in the answering of prayers. So, if I pray for healing for someone,,especially emotional and psychological healing, and it happens, it means that this was His Will for that person. If I pray for souls in purgatory, which I have been doing for years as part of my vocation of prayer, I find I get signs of these persons' release.This is part of my vocation of prayer and God's Will.
Such is the Will of God and the authority given according to my vocation.
If we are praying in the Will of God, things happen almost instantaneously. But, as in the case of St. Monica's long years of praying for her son, Augustine, God sometimes tests us to see if we are persevering. Also, we are purified through years of prayer for other people. We are purged of selfishness through these long prayers.
Look at the prayers God has answered in your life. There may be a pattern which indicates both your vocation and God's Will in your life and the lives of those for whom you pray.