There are situations we may not be able to generate a screenshot. If we're unable to find a matching ad slot on the url when we visit the page, we will not be able to capture a screenshot and you will see this result:
If you know that ad slot exists on that url, you may want to regenerate the screenshot(s). We may not have been served that particular ad slot during our visit to the url, so you may want to try to take a screenshot again. To take this action, simply select all ads you want to regenerate, and click Regenerate:
We hope this helps. If you have any other use cases of screenshots not being captured, please let us know and we're happy to help.