Hi Sarah and Shaun and Alex.
Well I'm late to the party on this one, but I came across your question while looking for something else. I don't know for sure, but it has to do with the popularity of your listing which is driven by the number of times it is viewed and subsequently booked/enquiries. If you have a long stay during a busy period, then your listing is not showing to all the enquiries being made for that period. Subsequently, the algorithims in Airbnb suggest that your listing is losing popularity. Further, other local listings are gaining in populairty and even more so relative to yours. So, when your listing does become available, it is possible that these other listings are being preferenced in the Airbnb search results.
I hope that is helpful, even if very late.