Why am I not getting a positive Opk

Laura

I’m so confused. For 8 months now my DH have been TTC and every month I get a positive opk on day 21. This month I went away for vacation so decided not to test until I got back. Today is day 21 and no where near a positive opk I’m so confused. I have a headache and cramps like I’m ovulating but nothing on my tests.