Ovulation help

So my opks have become darker and darker but not had a dark positive yet, yesterday it said nearly there on my <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a>, just done one at 6.50pm and it’s not as dark as previous pnes I’ve done, glow predicts ovulation day astomorrow ! Is it possible I have missed my positive?