PDF output showing blank spaces instead of math [closed]

Question

I have a big problem now. I need to print my thesis tomorrow, but I learned that for mysterious reasons my supervisor most of the time sees blank spaces instead of math symbols in the pdf I send her (and her printer's output looks the same). The symbols she cannot see are latin letters and numbers --- greek letters and other symbols look right.

I cannot test this more thoroughly and I do not know what computer she uses, but I am afraid that a problem like this could happen at the print shop. What scares me the most is that I don't have any idea of what's happening so if there will be a problem I won't know how to solve it.

I attach my preamble

\documentclass[12pt, openright,twoside]{report}
\usepackage{geometry}                
\geometry{a4paper}         
\usepackage{graphicx}
\usepackage{amssymb}
\usepackage{mathtools}
\usepackage{pgfplots}
   \pgfplotsset{%
        /pgf/number format/use comma,%
}
\usepackage{fontspec,xltxtra,xunicode}
\usepackage{siunitx}
\sisetup{output-decimal-marker = {,}}

\setromanfont[Mapping=tex-text]{Hoefler Text}
\setsansfont[Scale=MatchLowercase,Mapping=tex-text]{Gill Sans}
\setmonofont[Scale=MatchLowercase]{Andale Mono}

\newcommand{\rttensor}[1]{\overline{\overline{#1}}}
\usepackage{bm}

\DeclareGraphicsExtensions{.pdf,.eps,.jpg}

\usepackage[italian]{babel}

\usepackage{booktabs}
\usepackage[fixlanguage]{babelbib}
\usepackage{hyperref}
\usepackage[ddmmyyyy,hhmmss]{datetime}

\usepackage{subcaption}
\usepackage{chemfig}
\begin{document}
$p$
\end{document}  

I am using xelatex for the output and I make extensive use of Unicode for accents, symbols,... so I cannot switch to a dvi->ps->pdf workflow

I am asking you if you know of what maybe causing this problem and what would be some safe method to print out everything, even if not at the best quality, since it is more important to have content.

EDIT: Now it is a MWE. My supervisor wouldn't see the p in math style that is in the document