I cant really explain why but my last grow the same happened, one was really stunted but the others were growing fine. I personally just deemed it to be a bit of an "handicapped plant". That said, I would keep it going if possible just for the experience, it actually surprised me how well it performed considering its messed up start, it was really weird in its grow pattern, growing in odd intervals with how it branched out and all sorts of interesting stuff. It was actually my most yielding plant that grow, although the weed was not as strong as the others. Hope your grow goes well mate! :)