Check to see if Google Feedfetcher is still showing up as a client. I
had one Feedburner feed (only one, though) that dropped by a lot
because, for some reason, Google Feedfetcher and Bloglines stopped
being able to read the feed, so this could be the problem you're
having as well.
A question for those of you having problems: are you using forwarded
feed? That is, your subscribers would subscribe to
example.com/
myfeed.xml, which your server then redirects to
feeds.feedburner.com/
myfeed?