Negative OPKs

I've been having negative OPKs all week. Started the day of the start of my fertile window and 7 opks later,still no positive. Now glow just keeps moving my fertile window forward. I only take the opks at night when I get home from work,does the time of day depend for the opks that I could have missed it? Or am I just ovulating late this month? Have a 31 day cycle and I'm on CD19. Please help,new to this ttc thing!