So I understand you are looking for Allegiant Air flights. I see that FlightAware has some schedules for this AAY to/from Pittsburg KPIT and New Orleans KMSY, but none from KLAS.
Unfortunately this airline may not have a good coverage being a low cost one. The team is trying to improve the schedules, but I am afraid there isn’t any quick solution for you.
That page shows flights today/airborne, they come from multiple data sources, including the ads-b network. Schedules are provided by other third party sources that publish them months before departure. We have contacted them regarding Allegiant Air.
Hopefully it improves. Thanks for letting us know about cases like these.