Ovulation

So for the past two months I have been <a href="https://play.google.com/store/apps/details?id=com.glow.android">tracking ovulation</a> with OPKs and I hit my peak on CD 15 , now this month it’s CD 16 and I’m still getting negatives. And to think I wasn’t gonna use OPKs this month because I thought I had it all figured out. Has anyone else experienced this? If so is it normal?