Help!!! So confused!!!

Hi all! So I’ve been testing with opk for about a week now... They started out all negative (no faint line or anything) and now that I’m getting closer to my Ovulation date I’m noticing that the line is getting darker. SO I’m testing twice a day, this is where it gets weird.... a few times my second test of the day is completely negative, no faint line or anything. WTF? Anyone else?