I would start taking opk from about cd10, just incase I missed it! You will get a positive before a temp rise, once you see a temp rise on your chart you've probably already ovulated.
I'm not sure about them not working for everyone, but some do have more luck with them than others. With opk you will always get a second line but it needs to be as dark or darker than the control line for it to be a positive, and indicates ovulation will occur within 12-48 hours. It's also recommended not to use FMU and test around 1-2pm.