Why do some YouTube videos show a thumbnail when you hover the mouse cursor over the time bar, while others don't?
Had you said previews on the progress bar (or even had uploaded images) I might not have misunderstood first time around. IF that is what you mean:
The progress bar preview works on some videos and doesn't work on others. There is no explanation for it, nor can you fix it.
from YouTube Help Forum might be of interest (though I'm not convinced correct).
It might probably be the format in which the video was uploaded. For instance where the script used in handling the thumbnail function can only read certain formats of videos like mp4