A poor supply of water or nutrients can cause a plant to be unfruitful. In the Christian life if we are not allowing God's truth to feed us, if we are not spending time hearing God's Word, reading it and allowing it to feed us spiritually we will be unfruitful. Insects and disease can destroy an otherwise healthy plant. Spiritual disease is called sin and left untreated it will destroy your spiritual vitality and productivity. Many people think that they can allow a little sin to remain in their life and it won't have any impact.
New branches require several years of pruning before they can produce.
This can be a natural process if we are young in the Lord. But we can also live in a state of perpetual immaturity. We need to give attention to our spiritual growth that we might move on from immaturity. Unfortunately, we are not the passive participants in the pruning process that a vine is and sometimes we resist God's attempts to focus our priorities and our energies on the things that will result in fruitfulness. We must allow our lives to be shaped by God and His Word, not by our own wisdom and desires.