OPK help please

This is my first month using OPKs, I have a very regular 27-28 day cycle and I’d like to pinpoint ovulation. I’m supposed to ovulate on CD 13 or 14, but today is CD 12 and I’m still getting hard negatives. I’ve tested on CD 10, 11 and 12 (today) so far, is it possible I missed it?

I’m also wondering if tests get progressively darker or if they can fluctuate up and down until there’s a clear positive? I feel like I should have at least a solid looking test line at this point but it’s barely there 😕

Thanks!