Ok, I will try to explain the issue. First of all here is my under construction website http://www.valbonne-team.org
As you can see in the sidebar I have several categories with parent and child for some of them. The issue I had was that when I click to "Aventures" category I only see post from "Via Ferrata by night" but not from "Pyrenees2005" => Strange!
So, I have checked several things and things appear to be quite strange :
Check the url link of the 2 category child :
Via Ferrata by night is ".../category/aventures/via-ferrata-by-night/"
Pyrenees2005 is ".../category/pyrenees2005/"
and SHOULD BE ".../category/aventures/pyrenees2005/"
So I have checked in the admin panels and the category appears (as in my webpage) well categorised between parent and child but when I edit most of the child category the parent item is filled with "none" ??? and when I try to change it to the right parent name and re-edit again, still "none"...
Really really strange behaviour with probably a nice bug in it....
Don't hesitate to ask precision if it is unclear (difficult to describe the issue)