Question. Cm vs. LH surge?

I recently spoke with my doctor about making babies and she suggested that I use opk's to see if I'm actually ovulating. So, I went to the handy dandy calendar and saw my projected fertile days and began testing the day before. I got a bunch of negatives, and I'm using the clear blue digital so it's really hard to misread. You either get a smile face or a blank circle. I went through my whole window testing and not one positive, meanwhile, during that time I had watery mucus and some egg white cm. Finally, yesterday, the day after my projected window ended, I get a smile face. By now o have the paste like white cm.  Is there a reason for this? Has it happened to anyone else?