Monthly Archive incomplete, only shows some posts

Hi all,

I have a monthly archive on my page (http://www.achteminute.de which somehow doesn’t show all entries, but only a partial selection. The monthly archive widget on the right side show the correct number of articles for each month (checked with backend), but once you click on a month, only some article is shown.

For example, the (annual) archive pages http://www.achteminute.de/2013/ are pretty complete, while the archive page http://www.achteminute.de/2009/ only shows one post although there are many. The page for 2008 http://www.achteminute.de/en/2008/ even doesn’t show any results although there definitely are posts (such as this one: http://www.achteminute.de/20080824/vdch-mv-3-bericht/).

The more I go back in the article history, the less posts are actually displayed, although I can manually view them via the backend.

Is there something like a hidden cache? Or a hidden attribute that I don’t see that all hidden articles might have in common?

Thanx for your help