previews in finder missing

Now, this is a bit of an old issue (and occurred in tiger too):

Some document (jpgs, pdfs, pngs, docs, gifs) icons in finder do not have previews, regardless of whether or not "display icon preview" is on, and regardless of the state of similar, or even copied versions of the file.
Some folders are better/worse than others, and some have complained about problems involving externals, and perhaps permissions?
Additionally, I've had instances when previously previewable files have, over time, lost their previews, sometimes while they've been in a folder I was poking around in. These previews can even go missing in the Cover Flow view and QuickLook, and just show the generic icon picture.

I really don't think this is due to processing power, or taking time/RAM to compile the previews, and I haven't noticed the correlation some have between PC origins and lack of preview.
Is there some way to force finder to rebuild it's preview database? Does anyone know what the deal is, or indeed, if there is one?

(I had a look around, and there were a couple o' threads, in this site and others, and an archived support article which didn't really answer it, despite the "answered" tag.)

Basically: some docs don't generate previews, or lose the ones that they get, does anyone know why?