Thumbnail generation in Ubuntu

I can generate thumbnails in CentOS by installing unoconv libreoffice-headless vlgothic-fonts ImageMagick.

However, I can’t generate thumbnails of PDF and MS Office files. Image files such as BMP, JPG is OK. I have installed Libreoffice, Imagemagick and convert command is running in terminal. What am I missing?

BTW, There is no English manual for

Thank you in advance.

Did you check fess-*.log files?
It’s better to check these commands in PATH environment.
Fess generates images by generate-thumbnail.