Positive OPK or just sitting too long?

Lisa
Grrr...according to glow I'm in my fertile window. No positive OPK's. Just took one around 4:00 and it seemed a bit darker than normal but not matching control. I happened to let it sit and it seems much darker now-pretty much same color as control. Am I getting closer? Or after leaving them for over the 10min mark do they get darker?