Category Pages Doesn't Show All Posts

Hi there!

I startet a blog some weeks ago and now recognized that the category pages don’t show all posts of the category. Please have a look:
https://mobiwatch.de/tests/

There should be for example a HTC One S which is DEFINETELY connected to the category “tests”:
https://mobiwatch.de/test-htc-one-s/

Is it possible that the category pages miss a “forward” and “back” navigation? Because I counted exactly 10 posts on that page which is the number I told WordPress to show per page. It might be coincidence, but I’m not sure.

Can somebody help me with that, please?
Thanks in advance!