Fares don't last forever sometimes even waiting 2 hours to book, you will find the space gone. You need to jump on them right away. Fares are filed and pulled several times a day. If only a certain number of seats are available at the price, they can be bought in an hour or 2 or even less.

One way is also usually more expensive than round trip as you need to meet a minimum stay for the cheapest fares.