I'd go with... "it needs more light". There may be some truth to the light argument. The growth does look a little strange. I've seen a couple 15 week sativas look like that 5-6 weeks in. I wouldn't even know if you could keep a plant so confused it didn't know if it was growing or flowering. How long has it been flowering and can you upload a picture from a couple weeks ago?
10/14 on/off will just quicken your finishing time. Please tell us about your light source. 12/12 is just a standard people use. You can flower Indicas easily at 14/10 on/off. I'd recommend 11/13 on/off for landrace sativas for many reasons. Depending on your light source, you want to give it as much light as you can in each day/night cycle. If your high powered, 12/12 or 11/13 on/off is fine. If your underpowered, I'd recommend 13/11 or 14/10 on/off for indicas. I just did a journal at 14/10 and I had a pure Sativa Bubblicious flower beautifully.
Good luck! Tell us about your setup.