I have 6 items but only 4 items are displayed instead of 5. 5 items are marked for publishing.

What can be the problem?? www.hgcars.be -> wagens

Hard to say without knowing more. Maybe you have a limit on that category display in the Category view options somewhere? As in, only set to show 4 items. Remember that if you have that category set to inherit settings from a different category, NONE of the settings you apply will work (they will be overridden by the inherited category options). So even if that Wagens category is set to show 20 items, but you're inheriting from a Cars category that is set to show 4 items, it will only show 4 items.

Thanks for the answer.

I have only 1 category : wagens, so no parameters used from another category.