Isn't faith either all or nothing? Why does Jesus say to the disciples "Oh ye of little faith"?
The word "faith" can have different shades of meaning depending on the context in which the word is used. In some contexts, the word faith refers to a believer's saving faith in Jesus Christ, while in other contexts the word refers to a believer's willingness to walk confidently in that belief.
In the first case, faith in an all-or-nothing proposition. A person is either saved or not. They are either believing in the promised Messiah (Jesus), or not. There is no third option. So in that sense, saving faith is all-or-none, because a person can't be "almost saved" or "almost believing" no more than a woman can be "almost" pregnant.
In the second case, a person who does have faith in Jesus Christ (and therefore is saved) can still show degrees of faith in how they live out what they believe. Some days a believer will demonstrate great faith and obedience while on other days we will not. This is the way Jesus was using the word "faith" when He declared that His disciples had "little faith." He meant they were not walking out what they believed. Likewise later in the gospel account, Peter sank into the Galilee because he was not walking (literally) according to what he believed about Jesus. These are examples of lacking faith in the sense of lacking a walk of obedience and confidence in God.
Similarly in Genesis 32 we see Jacob's lack of faith as a lack of obedience. Jacob was a believer, so we know he had saving faith, but he was also a man who showed weakness at times in walking out what he believed. His saving faith was 100%, but his walk of faith varied by degrees. Some days Jacob showed strong faith in his walk with God, while other times he displayed weak faith, but in all cases he possess 100% saving faith.