When do we need to have sex when using opk?

So I’m taking Letrozole this cycle and usually get a strong positive on day 12 and even stronger positive on 13(even without the Letrozole). We’ve been trying for 4 years now and it’s been over a year since our last miscarriage. I ovulate every month and don’t understand why I haven’t gotten pregnant....We usually hold off doing anything for a few days before I get a positive opk and then have sex that night and the next night and then every other night there after. Should we be having sex before I ovulate? Wish getting pregnant was as easy for me as it is for so many other. All advice is appreciated.