Hello @Tina238,
One possibility is the instant book setting. Airbnb is pushing instant book. To this end they sometimes turn that filter on when people search. I've found that they turn it on automatically if I search for dates that are in the near future or for an area where there are a high number of listings. In those cases I've to turn it off to have it show non instant book listings as well.
Could it be that searches where your listing didn't show up had the instant book filter turned on automatically?
Also, it could be that for a second search with different dates there are more listings available in the area for the second search so you may have to zoom in for your listing to reappear. That happened when I first searched for the 1st to 16th of September. Your listing showed up then but disappeared for a search for the 16th to 31st of October. I had to zoom in one notch for it to reappear.
Steve.