I propose to have a look at this MnWE :
\documentclass{article}
\usepackage[eulermath,nochapters]{classicthesis}
\usepackage{lmodern}
\begin{document}
$$ (x+2)(x-5)$$
\end{document}
When compiling (pdflatex), the parenthesis do not appear in the pdf. The problem disappears when commenting either lmodern, either classicthesis, either only eulermath. What work around would you suggest ? Thanks! S.

classicthesissets the default font to Palatino, loadinglmodernbefore it has no effect whatsoever. – egreg Jul 16 '15 at 10:52