In which situations would the thumbnail of an attached document not render on a task...

Alfresco Process Services cannot find a license for Aspose, the 3rd party library being responsible for document previews and thumbnails.

The uploaded document is larger than the allowed maximum file size of 5MB.

The uploaded document has a MIME type that cannot be rendered as thumbnail.

Elasticsearch is not configured/installed properly.

The form page has to be refreshed in order to render the thumbnail.