I have the same issue. I raised up the unpaid payment 2 months ago, but no progress at all. I had sent several reminders, and called customer experience, but no reply to my reminders, and customer experience just say "the case is under investigation and the professional team will respond you soon after it is resolved". I think the longer time passes, the more difficult I can get the payment.
According to customer experience, Airbnb had received the payment when the booking was initially made, but guests advanced check-in date by day, causing an extra charge for 1 night, which had been left unpaid by guests. Airbnb cannot pay to hosts until they receive this unpaid part.
I think it is very primitive yet fatal system/structual issue. Airbnb accepts booking only after they assures the payment. The same principle should be applied to the booking change. Otherwise, guests can make a booking for 1 nights, Airbnb/hosts conrifmed, then guests can change for 30 nights, don't pay, but hosts cannot notice the unpaid portion.
Customer experience says over the phone that there're lots of such cases so it may take time for investigation. If it is so, the system should be changed immediately, but it seems no action. It is because Airbnb does not get any damage from it. I wonder how much of "unpaid" money has been suspended in the whole world...