Fertile cm but negative ovulation test?

I’m day 15 or 16 in my cycle, i noticed i’m having fertile cm all day, I took an ovulation test and it was negative :( I read on the box that it detects a couple days before.. could it be that i tested too late but i’m currently ovulating? or how does this work?

IM VERY NEW TO OVULATION TESTING.. please be kind 💜