Opk not positive

I have an average 28-29 day cycle, and I started taking opks about two days after my period stopped. I take one around lunch time and one at night time just incase, but I still haven't got a positive yet. I'm on cycle day 17 and theyve been very faint every day except for CD 14 but even that was only half as dark as the control line and still fairly faint.

I'm only taking bulk opks I bought off ebay, do you think maybe they're dodgy or I just havent ovulated yet? Or maybe the semi dark one was it I'm not sure.. I'm getting worried :s