It turned out I was ovulating 3 or 4 days later than I thought I was and even though we dtd quite a lot it's easy to go at it hammer and tongs at the first sign of ewcm then run out of steam a bit and miss the actual window!
Don't stress too much about dtd on ov day, on the FF app you can look through loads of other people's charts and a lot of BFPs come a day or two, occasionally 3, before ov day. You get confirmation you've ovulated 3 days after, so keep going every day or every other day till that happens.
I've got quite nerdy about it and DH likes a visual snapshot of what's going on, who doesn't love a colourful graph! The free version is fine but if you want to keep track of a lot of symptoms, you might like the paid for version, it's so much better than Ovia, which I used for a while and is all guess work on the app's part with some stuff about cm and cervix position, where temping gives takes in actual data.
I've recently miscarried so getting back on it in the next week or so hopefully and while temping doesn't tell you when you're going to ovulate (I really can't be arsed with opk though a lot of people love them) it's like a tiny pat on the back to have it confirmed after the fact. Looking back, I can see that my BFP chart was very different to the ones before, like kaytees says. Even more useful than that though is seeing it not having happened in a month, or that my cycle was shorter or longer based on my ov date, so I didn't get my hopes up about being "late".
As the book explains so well, you don't get a late AF (unless you're pregnant when it won't show up at all), you get earlier or later ovulation, and the time between that and AF (luteal phase) stays the same within a day or so.
I saved a small fortune in tests and the hassle and stress of early testing as I got to learn my cycle and had an idea of what to expect.