There is a bug with the Instant Book feature. It affects my listings and many others also, but some listings are not affected. I have reported it to AirBnB but do not know if they will fix it. Here is the problem:
You have Instant Book enabled on your listing. You can search for a few nights stay for your listing and see the lightning bolt next to the price under the listing photo in the search results which confirms Instant Book is on. When you change the search dates to be 15 nights or longer, the lightning bolt disappears on some listings meaning the AirBnB system thinks you DO NOT have Instant Book enabled on your listing.
Since the default search now has the Instant Book filter in the ON position, your listing may not be showing up for guests wanting stays of 15 nights or longer. I would say this bug affects more than half the listings, not just mine. In the case above, 24 listings have the Instant Book feature on, and 15 of them are dropped off the search results when changing from a 14 day to 15 day stay.
It is not obvious to me why the Instant Book feature stays on with some listings and not with others when searching 15 days or longer bookings. I don't know a good workaround. Having Instant Book enabled on your listings seems key to getting high search results and more search results so I would suggest leaving it enabled, but realize the search results may not be what you expect.
maybe if they have maximum stay less then 15 days? So for longer stay you should ask first (wild guess)
Yes, you have found the solution. If no maximum stay is specified, the AirBnB system makes it's own limit of 14 days for instant book. So the workaround to avoid this bug is put in some value for maximum stay eg. 60 days, 90 days, etc and then Instant Book remains on until the maximum stay value is exceeded.