If it's an error, then it can be fixed. If it's simply that there were only a few of the lower fare bucket available and someone purchased while you were still looking, that is the nature of the business. Doesn't matter whether you miss the opportunity by 0.005 seconds or 5 days.

To put it in perspective, in 2018, WN carried 138.9 million passengers. At any time of day or night, there are a lot of people online.