the 0.5 is made for 15w-30w and I've heard of people vaping it at 13/14w as well but I find it interesting that you actually get any vapor out of it with 9-10w
it could be that you got a bad batch of coils. what mod are you using your subtank with? check on the screen of your mod if the resistance changes during use. like when you first fire it it is somewhere around 0.5/0.6 and then it spikes to 0.9 or higher and then back to lower wattages?
I'm using the kbox mini and the kanger 0.5 ohm OCC, the resistance remains constant throughout at 0.5 ohm, no spike at all