Hi Mooshie,
I have learnt that many many women just don't get a true positive but still ovulate (Me!).
I used Fortel and never got anything close to a positive but serum progesterone tests 7 days after these second lines showed I had ovulated.
I changed to Lullaby opk's and I conceived both cycles of using the opk and ensuring I bd when I got a second line. Lullaby's second line is darker and most women find a second line heralds ovulation. You will find you will get to know your body and will get a feel for what the test results mean for you. As a rule of thumb thouogh bd when you see a second line and the day after as well.
Remember that the opk is picking up the surge. This surge is the hormone surge that triggers your ovaries to release the egg. So, you ovulate after the surge, 12-36 hours after.
Try to hold your urine for at least 2 hours and don't drink for the couple of hours prior to testing.
I wish you all the luck in the world at catching a healthy egg!
love Deb