Lack of darkening lines??

I’ve been taking opks since my period ended, and today is cycle day 16 for me and I’ve yet to see any darkening lines to indicate a hormone build up... last month I had a dark line on cd 16, and my positive came on the 17th day. Can a positive opk just pop up one day without any prior line darkening? Here are cycle days 14, 15, & 16 (today) for this month.