Dang! I just read an article on the main site about when one in a group doesn't flower. Can't seem to find it. There are a few reasons and only a possible solution.
When a marijuana plant does not look like or do what a 'typical' marijuana plant does, especially when it's one of several in a group, blame genetics. From a seed that doesn't pop to an extra-large producer that has double the suggested THC-A percentage. If it one of these things just ain't like the others, it's genetics.
Another possibility is that it is not the droid you are looking for. I mean... it's not the seed you think it is. Maybe there was a mix up.
A solution is to change the light cycle to 12/12 and flower it like a feminized seed, not auto. See if you can force it to flower.
But, my opinion, when grown in the same environment, they only thing you can really say when a plant doesn't do what it should is it's a freak, mutant, or other genetic mishap.
Have any pictures? Could it be a male?