Is there a way to debug this?UPDATE: it just started working.
Why does Google preview gives preview not available for site (even though magnifying glass is shown)?
google
It probably means that they haven't had a chance to make a preview image yet. I would guess that Googlebot and the image generator are separate processes and have different sets of priorities and queues. I wouldn't worry too much about it. Wait a week or two and if it's still the case, it might be worth posting a question on Google's forums.