I found the flight on your website, placed an order, but have received an email saying that the flight is sold out.

Prices can update throughout the day based upon customer demand, fluctuations in availability, and airline allocation. The general rule says: the sooner the departure date, the quicker the changes. Some people wait till the literal last minute to book their flight – when only a few seat left. Those seats “disappear” (are being booked) in a matter of minutes.

Tip: Always book your ticket at least few weeks in advance. It also guarantees you the best price, as the airline still have a good number of seats on offer.