Artwork not sticking ..

[dtsig]

using 1788 but this is not a new occurrence .. been trying to work it out for a while.
I use calc cover size to get the size of the artwork attached to a file.
A Clockwork Orange shows as having no artwork. Shift-Enter, Alt-T, A .. I select a jpg in the folder with the same name as the movie file. The image shows in the Art & Details/Selected.
I then Calc Cover size and that works.
I arrow down off the file then arrow back up. Artwork is gone. If I Calc Cover size again it clears out.

Why do some files refuse to accept/keep artwork? Files are either mp4 or avi ... problem with both types.