clearblue OPK help please

First month using clear blue fertility monitor Thai month and so far I'm on day 10 of high fertility. I had what I thought was a positive opk on CD 13 and CD 14 and they got lighter but then on CD16 I had a really dark positive opk but clear blue still says high. I'm on CD18 today and cheapies are really light. from CD13 to CD15 I had sharp cramps and spotting which I though was a really clear indication of me ovulating which has now gone. What do you ladies think? I would have though that clear blue would be more accurate as more expensive but I'm now starting to wonder if it's a complete waste on money