trouble with Template File Hierarchy

Hi, looked at many info in this forum, but I cannot find my problem with the archive.php: it’s not showing, instead it switches to index.php.

The “normal” Template File Hierarchy is:
category-#.php
category.php
archive.php
index.php

I can and maybe will make a category.php for my theme and place some coding outside of the loop as described in the Codex (Different Text on Some Category Pages).

But before I do that, I would like to understand why my theme does not pick up on the archive.php which is present and instead takes the index.php?

The only reason I can think of is that my categories have no content yet…

Can someone confirm this please? It would help me a great deal!

Thanks,
Piet