Change of thumbnail and picture size

I don’t see anything obvious in the mediainfo that could cause this then.

You should consider setting up a library. Works far better that sorting through a bunch of directories. Here’s some good information to get you started: https://kodi.wiki/view/Video_library

Hi and thanks for the link on Video_library. Will need to think about this as we have several options and much not tagged.
To close this thread my problem was not with Kodi but most likely with an earlier version of ffmpeg. It has been easier to re-save the video in the offending cases.
Many thanks.