"If you have faith as a mustard seed, you can say to this mulberry tree, 'Be pulled up by the roots and be planted in the sea,' and it would obey you."
Luke 17:6
Jesus told them that it wasn't an increase of faith that they needed.
What they needed was to apply the faith they already had.
Even if they had faith only the size of a mustard seed, it would be more than enough to do the job for them.