Actually, I'm not using the default permalink structure. I'm using one that uses the post title in the URL.
A friend has just written me to say that unless your individual posts are well-linked to by external sites your PR is not going to be very high. And I don't think many of my individual posts are widely linked in that way so that prob. explains the '0' ranking for most individual post pgs.
don't worry about it. I had it happen on 2 of my ESTABLISHED sites. dropped from PR 5 and PR6 to PR 0. then PR came back on homepage, now it is on homepage only but not on subpages. Just something that happens.