A few people have reported that some individual posts weren't posted to twitter from their feed, even though everything appears to work ok. There does seem to be an issue with twitter at the moment, where it sometimes returns an "OK" status code rather than an error. For us this means that twitterfeed is being told that the post happened ok, even though it didn't, and it will therefore not try to re-post it (as it would if it had received an error back from twitter).
There's a discussion on this here:
http://groups.google.com/group/twitte...
Hopefully twitter will fix this soon, in the meantime there's not a great deal I can do at my end.