Ovulation tests

Just wondering if anyone can help. According to my calendar on here my ovulation day should be Sunday. For the last 4 months I’ve been going by this app and working TTC around the days they’re saying. This month I decided to start doing OPKs and I’ve been doing them twice a day the last 4 days but some days are a lot fainter than others is this normal? I haven’t BD yet cause I was waiting for it to be a strong line just don’t want to miss this cycle ☹️ is it best to do as strips are saying or just BD like I have been? What if I don’t get a positive opk at all?