There are many reasons why God's blessings don't always come to pass in a person's life, but the most frequent cause is simply unbelief on our part. There are no limitations on God's part. He is willing and able to do anything He has promised. The problem is with our believing.

The Lord has given every believer faith (Romans 12:3). It was given to us at salvation through hearing the Word of God (Ephesians 2:8; Romans 10:17). It is a fruit of the Holy Spirit (Galatians 5:22). But, we must choose to exercise it.

Our faith is linked to our knowledge (2 Peter 1:1-4). What we think on is how we will be (Proverbs 23:7). The problem is not that we don't have faith. The problem occurs when we don't allow our faith to work because we don't keep our minds stayed on God's Word. Then our faith is choked out by the cares of this life (Mark 4:19).

Keeping our minds stayed on God's Word will release our faith and that will release the power of God in our lives. 

Mary must have been a person who put God's Word first in her life. She saw a performance of what she believed. We receive what we believe, not what we desire or even need.