Header missing on one page (first page of section)

A problem I am having is that I am missing a header on exactly one page throughout the 140 pages of my thesis and I can not figure out why. I am not using any packages like fancyhdr for headers – just pagestyle{headings} at the beginning.
This is how it is supposed to look:

My Latex file for this section looks like this:

section{Einleitung}
label{sec:einleitung}
subsection{Motivation}
Raumakustik ist mitunter eines der wichtigsten Themen im Alltag,
dem allerdings in vielerlei Hinsicht nur sehr wenig Beachtung...

This works for every section except for one, where I get these results – with the header missing:

But I still have the same code:

section{Grundlagen der Raumakustik}
label{sec:grundlagenRaumakustik}
Um gezielte Maßnahmen zur Verbesserung von Raumakustik umsetzen...

Is there anything I might have overlooked?
Thanks!

documentclass[10pt,a4paper,twoside]{scrartcl}%I used documentclass[10pt, a5paper, twoside]{scrartcl} in an effort to reduce time scrolling blank pages.
usepackage[cp1251]{inputenc}
usepackage[english]{babel}
%usepackage[left=2cm,right=2cm,top=2cm,bottom=2cm]{geometry}
pagestyle{headings}
begin{document}%pdflatex
section{alpha} Az.newpage
section{beta} Buki.newpage
section{gamma}subsection{delta} Glagol'. Dobro.newpage
Est'.newpagesubsection{epsilon} Gis'n.
end{document}