I honestly think everyone is different. There are lots of statistics flying around the internet that suggest certain days are better but I think it all comes down to the individual.
With me, I got my peak ovulation on CD17 (April 19th), OH and I dtd on that same day. I had implantation bleeding 5 days later (April 24th) and May 1st I got my BFP. They say sperm can live for up to 7 days but a more realistic figure seems to be 2-5 days depending on the mans sperm.
Are you using CB Digi ovulation tests or are you just using the strips? I'd highly recommend CB Digi if you can afford as it tells you when you're in high fertility (when beginning to ovulate), and when you're in peak fertility (when your egg is beginning to start its travels). Once you get your solid smiley face on the digi - I'd recommend dtd straight away because you have 24 hours to catch the egg before the egg dies.
If your positive opk is varying each month then you definitely need to keep track of where you are. I think it's all down to the luck of the draw at times too... I mean you seem to have been doing everything to the book so far - dtd on day of or day after a positive opk, but for some reason you're just not catching the egg. I really do think it's luck of the draw sometimes. We caught our egg first month so we were lucky - wish I could pass on that luck to you hun.
Just keep tracking your ovulation and dtd on the day of your positive and also I would recommend on the day after too.
Good luck xx