Sorry youve been trying so long, it took us a while too.
Id say if youre trying to bd after ovulation this could be your problem. You need to start bding 5 to 6 days before Ovulation so the sperm is ready and waiting for the egg to be released. How long are your cycles usually? Dont be afraid of the opks, you could use them just to notice a pattern of days your fertile and then stop if you have a pretty regular cycle, but if you can pinpoint your O day in your cycle you can then plan in cycles coming up your fertile days before Ov to bd.