Last week my guests left and until now I didn't get the prompt email to write them a review, but somehow we succeeded to write each other a review. Today I had some problem also...my guests checked out in the morning but they still on this trip until the afternoon, just a few hours before my new guests arrived.
I left a message to Airbnb on Facebook and then I got the prompt.
I managed to write a review to the previous guest without the prompt, through the page with our conversations. On top of the page I saw the message: You have 13 days to write a review.... I just clicked on the link and Icould write the review!