One of the reasons why most of our prayers are not answered is because of 'lack of faith.' And the Bible says faith is an evidence of things unseen.
For our faith to be active and work, we must step out from our flesh and begin to walk in the Spirit of God; begin to see things as the way God sees them.
Sometimes too, we feel that our faith is too small, or we not are too "Christianized". The scriptures says even if our faith is as small as mustard seed, it can move any mountain.
In that sweet Gospel of St Mark 11:22, Jesus says; "have faith in God".
Have faith in God means, "have the faith of God". The faith of God is all things are possible. This is the type of faith man was created with; all things were possible to man, but when doubt entered in him at the fall, (Genesis 3:1-7), it corrupted that faith.
When the Messiah (Jesus Christ) came, man was restored to that God faith in the new birth. All we need now as Christians is to exercise our little faith to grow. This can happen by hearing, and hearing the word of God. This why it is very important to read our Bibles.
We don't need to pray for bigger a faith, just exercise the little one you have with the Word of God and with prayer, for this how faith grows.
Finally, faith can only grow when there is forgiveness. Where there is no forgiveness, our prayers also cannot be answered.