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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