Hi,
I'm sorry to say this, but you've got a long road ahead if you think you'll get pregnant straight away by dtd once in your fw.. obviously it's possible, but if I only dtd once in my fw I would pretty much assume it wasn't going to happen that month. I'm not saying it can't, obviously it does only take one time, but you need to increase your chances.
The general advice is to dtd every 2-3 days throughout your cycle, and a little more in your fw if you know when it is. There is something called SMEP where you dtd every other day from CD8 until you get a static smiley, then 3 days in a row, miss a day and then dtd the day after for good measure, which you could look into.
I've just been reading 'it starts with the egg' which is a brilliant book but it is mostly aimed at people who have been trying a while/have fertility issues. However it does very helpfully explain the importance of egg quality. The basic premise is that sometimes we release an egg which isn't chromosomally balanced (there are missing or doubled up chromosomes). If this happens then no matter how many times you dtd, that egg is just not going to get successfully fertilised.
Sorry for a long response, but basically there are some months where it just won't happen. Don't beat yourself up that the one time you dtd in your fw you didn't get pregnant. I thought I would get pregnant straight away too, but I was dtd more frequently (spoiler alert - I didn't..!)