I believe like Peter when he was walking on the water to Jesus we take our eyes off of the Master. We start out all faith believing but when things begin to get even more difficult, we take our eyes off of Jesus, we forget His promises and we begin to think about the problem how big our problem is and we forget how big and powerful is the God we serve.
To have robust faith is to constantly declare the Word of God, reminding ourselves of what the Word says no matter the circumstances. Somedays doubt and fear may want to cripple our faith but even then because we believe Gods Word we still declare "we are more than conquerors " " God has not given us a spirit of fear but of power and love and of a sound mind" . It's standing on Gods Word no matter the circumstances believing that He is faithful and would never give us more than we can bear and that He will bring to pass His promises in our lives.