Opk fair line-may have ovulated?!

Kristin • Married 2014👰🏼 miscarriage October 2014🌈baby boy December 2015💙 baby girl due July 2020💕
I have gotten lines for my opk but it has never been as dark as what is suppose to be positive. I was researching and I found some info saying some women may not surge a high enough level to be detected in urine, but still ovulate. 
​Anyone else know more about this, or has happened to you?