Changing the feed won't cause them to flower.
If they really are autos, it won't matter if the hours are 16, 18 or 20, they should flower at all these light schedules, autoflowers flower due to age, not lighting schedules.
Since they are 51 days old, one would have expected them to at least be showing some signs of flowering and since they are not, maybe you should consider that they are actually photoperiod plants.