Thumbnail not showing for one particular .docx

[MC147]

A have a folder with many Word documents .docx - all show thumbnails and the Save Thumbnail box is ticked. However, one particular file refuses to show the thumbnail and just shows the default Word Icon.
As an experiment, I saved the document as just .doc and the thumbnail appeared. The diagrams in the document had moved around a bit, so it was of no use. When I converted back to .docx, the diagrams were correct again, but the thumbnail was missing again. Not a big deal of course, but this seems like a bug to me. I would have said that maybe some documents with diagrams are just to difficult to provide thumbnails for, but the experiment with .doc disproved that.

Anyone else experienced this problem, or got a solution - short of reinstalling Windows or Office or something like that.

p.s. Windows 10, with Office 365 - all up to date versions.

[MC147]

Found a solution to this problem myself. There was an image box in the document that was a .png image. I converted this to a straight forward .jpg and the Thumbnail miraculously appeared.
Maybe Word has a problem with .png images in the thumbnail page.