OPK Frustration

Hey ladies. I've been using a first response digital opk this month and I'm down to my last test strip for tomorrow with no positive. Anyone else have this? This is our second month trying and last month I got a positive opk. Really hoping I still ovulated somewhere in between tests. What's the latest you've ovulated? I'm CD19 and ovulated on CD16 last month. I've been in my fertile window since Monday and hubby and I have been BDing every day. Been having pretty significant cramping this week. Anyone else have this during ovulation?