Mrs B- I honestly think your BD timing is fine, actually. If you did it on day 18, which is how I'm understanding it, your chances are pretty good!
I think sometimes we get so hung up on the timing and frequency of sex in relation to our fertile window, that we forget how ladies throughout time have almost always gotten pregnant from one sexual encounter, at around the right time. When I got pregnant last May, we had sex twice that entire month, about two weeks apart!!!
So, anything can happen. I don't necessarily think that too much sex is great for conception either. Unfortunately, TTC is such a gamble...there's no real tried and true formula that works each time.