Why spend some clips endless time in processing mode?

Have some clips which stay in processing mode since days and cannot edit the uploaded batch in it's row.
What is the problem? some clips can be edited within minutes, others needs several days.

Yes, I noticed that to on big ProRes files. Maybe because of conversion from 4k to HD?

But I notice too, that a file that was "processing" since sunday is now vanished from my upload site.
What's happening?

It happened to me tot, but I think it is some kind of bug, because when I uploaded those "processing" files another time, the new ones and old ones were ready to edit all in one place...