Problem with Lightroom Standard Previews

I had posted elsewhere about a problem with the "standard previews".
Basically they can be there at one point and "POOF" they are gone the
next time I open a particular folder. (My settings were 1440 and
Medium.) Even within a folder there can be some that show the preview
fine and some show previews that look like a very small JPEG file (and
badly compressed to boot).
It was suggested that generally closing LR and restarting it generally
solves this problem - nope did not help.
Any suggestions - aside from deleting the files and importing them all
over again?
Thanks,
Steve