latex: why the \sum Symbol will disappear?

I am really confused why the \sum symbol will be disappeared after a few times of compiling. It was normally displayed previously, but now it appears as the following:

where the denominator in this equation is a sum from i-1 to n of AIan

My current packages used are as followings:

\usepackage{booktabs} % For formal tables
\usepackage{amssymb}
%\usepackage{amsmath,amsfonts,amssymb,amsxtra}
\usepackage[ruled]{algorithm2e} % For algorithms
\usepackage{verbatim} % For comment
\usepackage{multirow}
\usepackage{threeparttable}
\usepackage{color}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[utf8]{inputenc}
%\usepackage[ngerman]{babel}
\usepackage{graphicx}
\usepackage{subfigure}
\usepackage{amsmath} 

Could anyone help me fix this problem? I have tried many different solutions, but the sum symbol still could not be displayed normally.

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