Easily force reloading the thumbnails by simply
touching the file(s).
Make sure you
cd to the folder first.
If you want more control on what you
touch (eheh), just update the glob to taste, e.g.
No need to put your hands in automated configuration folders.
The problem often happens because the thumbnail manager is called as soon as the file is created, often fast enough that it is not completed yet. When creating (encoding videos, creating plots, merging documents, etc.) large files, the thumbnail manager may (try to) create the thumbnail (and fail) before the file is complete.
touch updates the 'last edit' time. The thumbnail manager finds the thumbnail to be obsolete (you 'edited' the file since it was last taken) and updates it.
EXTRA: if you happen to need more control on the files (e.g. include subfolders, file patterns, etc.), you can use something like this:
find . -name '*finished*.mkv' | while read f; do touch "$f"; done