Preview not available

Sometimes the PDF preview is not available. Closing the window, opening new one heals it. any ideas?
v 3.8.7 on Mac OS 12.6.2

Does this mean that there’s no preview pane at all or that the selected PDF is not shown in the preview pane? A screenshot would be useful.

thank you …

preview by hitting space available, no problem

No thumbnails will be shown unless the view/edit pane is displaying the document. Choose Standard or Widescreen from the View menu.