Hi - I hope someone can help

When I look at my phone image files through my laptop via USB MTP using Windows10 I can see some file thumbnails as images (those from the DCIM Android folders - DCM images and other JPG images transferred to my phone from my camera) but other folders e.g. the Aviary folder, or other third party app adjusted photos, or Whats App images (in IMG format or videos) all show as the generic icon rather than thumbnail image (weirdly there's one out of hundreds of Whats App files that shows as an image (????!!!!). This means file management is virtually impossible.

I've googled this obviously and can see

a) that my Folder Option / View / 'Always show icons never thumbnails' is unchecked, and
b) that through advanced system performance settings the 'adjust for best appearance button' is pressed and the 'show thumbnails instead of icons' is checked

What am I missing? Can anyone help me resolve this -
Thanks in anticipation