@LeonidS videos were in Mov and MP4 formats and sizes less than 5 mb.

This issue happened after I uninstalled and reinstalled Timeline module (RC4). It affected, for some reason, only 3 video posts on timeline but multiple images (JPEG).

Since, I couldn't find a good answer, I ended up manually chasing these videos on the db, then uploading them again (nothing changed!) and now they show and the Transcoder log is cleared. However, for the images, it will be impossible to find them and change them, therefore I forgo that (unfortunately).