Opk problems

I am looking for advice on opks. I have been using clear blue digital and they have failed me twice. This month I took femara and we were supposed to do an IUI. Well, I never got a positive opk even though I ovulated on day 17 (I can tell from bbt). So we missed out on IUI. I did clomid back in January and I got a positive opk 3 days before I ovulated, so we did the IUI at the wrong time. I am so frustrated and looking for help. Should I try wondfos instead? Thanks ladies!