I have discovered that at least 3 of my older articles are not showing up in their category view :(

However, they could be found when browsing their parent category (with "see child items" enabled), also they appear in list of last modified, when I re-save them.....

But where are they gone from their category view?

Does this category maybe have set to show only the featured items?

No.
It seems like I solved it - but I'm not sure yet what was the cause...
I had also SH404SEF and K2 items in categories where displaying like Page-1.html Page-2.html etc.....So I deleted those URL's from sh404 and refreshed a page.....They were there...