If that doesn’t work you may check ~/.config/dolphinrc
The preview settings here (15.2) are this:
[PreviewSettings]
Plugins=imagethumbnail,jpegthumbnail,kraorathumbnail,opendocumentthumbnail,directorythumbnail,gsthumbnail,rawthumbnail,svgthumbnail,textthumbnail,ffmpegthumbs
You can compare the working / non-working settings of your both users. Any differences?