Hi there.. to say it as it is: autoflowers are a genetic mutation in indica and sativa plants, that have been "taken over" from the ruderalis family.. unfortunately the ruderalis family are small, low on thc and don't have a big yield, so they're not very interesting for most growers.
To get these autoflower-traits in a plant that has the other traits of the Indica or Sativas, they need to be crossed multiple times via multiple generations, to remove the "photoperiodic" trait (which to my understanding is a dominant trait) from their genes, and substitute it with the "autoflower" trait.
This however doesn't have to be perfect with each seed.
In case you're really interested, i suggest reading up on Mendel's laws of inheritance, his law on dominance and genetics in general..
It might just very well be, that this one seed ended up being a photoperiodic one (it has happened before).
Try to put her under 12/12 and see if she then starts to flower.
happy growing!