[Users] "Mark as read" in RSSyl folder is unpredictable

[Cliff Laine]

I subscribe to about 90 RSS feeds. Some new posts, for reasons I
can't work out, always show as unread when I go into the folder, or
open Claws, no matter how often I mark them as read. One such feed that
always shows unread is a Blogspot Atom feed -- surely that should be
standards-compliant?

I'm sorry about the lack of detail but I can't identify easily and
distinct characteristics of the ones that misbehave.

Has anyone encountered this before, and found out a likely cause?

Thanks

Cliff
CM 3.8.1 
Linux Mint 14 Nadia