Strange behavior of Rss Feeds

I noticed a strange behaviour in the Rss Feeds of my Roller
2.3
installation: Sometimes, in condition that I cannot define,
the main
rss feed for a weblog (./roller/rss/blog) won't update. All
the other
feeds (including categories feeds, or feeds with a
"flavor" parameter)
do update, but the last one doesn't update; and does show a
lastBuildDate dating from the post before.

Any Idea on what might cause this ?

Thanks in advance

-- 
Axelle Ziegler