NB! I Corrected the \label[tab]{results}, which is wrong syntax unless your thesis-package redefines the label-command. However, I really recommend you to have a look at cleveref.
Use tabular X-column since you are using tabularx. In addition, I used a fixed width column as first column using array’s w-column (require an updated version of array). Also, use booktabs rules instead of hline and cline to have better spacing between the rows. If you prefer more space between row 2 and row 3, use \addlinespace from booktabs:
Example 1
\documentclass[12pt]{article}
\usepackage{tabularx, booktabs, array, caption}
\usepackage{multirow}
\begin{document}
\begin{table*}
\centering
\footnotesize
\caption{Parameter settings and results for different scenarios.
\label{tab:results}} %%%% <------- Corrected
\begin{tabularx}{\textwidth}{@{}>{\raggedright}wl{1.5cm}*{12}{>{\centering\arraybackslash}X}@{}}
\toprule
\multirow{5}{*}{\begin{tabular}[c]{@{}l@{}}Part\\ Type \#\end{tabular}} & \multicolumn{12}{c}{$\mu_j=U(91,876)$}\\ \cmidrule{2-13}
& \multicolumn{3}{c}{$\lambda_j=U(0.01,0.1)$} & \multicolumn{3}{c}{$\lambda_j=U(0.09,0.1)$} & \multicolumn{3}{c}{$\lambda_j=U(0.1,0.5)$} & \multicolumn{3}{c}{$\lambda_j=U(0.5,0.9)$} \\ \cmidrule{2-13}
& \multicolumn{12}{c}{Planning Horizon (Years)}\\ \cmidrule{2-13}
& 10& 20& 30& 10& 20& 30& 10& 20& 30& 10& 20& 30\\ \midrule
1& I& I& A& I& A& I& A& I& A& A& I& A\\
2& I& I& A& I& I& A& I& I& A& A& A& A\\
3& I& I& I& I& A& A& A& A& I& I& I& I\\
4& I& I& I& I& A& A& I& A& I& I& A& I\\
5& I& P& P& I& I& P& A& I& P& P& P& A\\
6& I& A& A& I& I& A& A& I& A& I& A& I\\ \bottomrule
\end{tabularx}
\end{table*}
\end{document}
Example 2
This is with split cmidrule and !{\hspace*{0.25em}} to increase space between column 4 and 5, 7 and 8 and 10 and 11 to group columns. In addition, I used \addlinespace to increase space between row 2 and 3.
It is the (l) statement in \cmidrule(l){2-4} that split the rule. Just remove the parenthesis and their content to have a solid rule.
\documentclass[12pt]{article}
\usepackage{tabularx, booktabs, array, caption}
\usepackage{multirow}
\begin{document}
\begin{table*}
\centering
\footnotesize
\setlength{\tabcolsep}{0.4em}
\caption{Parameter settings and results for different scenarios.
\label{tab:results}} %%%% <------- Corrected
\begin{tabularx}{\textwidth}{@{}>{\raggedright}
wl{1.5cm}*{3}{>{\centering\arraybackslash}X}
!{\hspace*{0.25em}}*{3}{>{\centering\arraybackslash}X}
!{\hspace*{0.25em}}*{3}{>{\centering\arraybackslash}X}
!{\hspace*{0.25em}}*{3}{>{\centering\arraybackslash}X}
@{}
}
\toprule
\multirow{6}{*}{\begin{tabular}[c]{@{}l@{}}Part\\ Type \#\end{tabular}}
& \multicolumn{12}{c}{$\mu_j=U(91,876)$}\\
\cmidrule(l){2-13}
& \multicolumn{3}{c}{$\lambda_j=U(0.01,0.1)$} & \multicolumn{3}{c}{$\lambda_j=U(0.09,0.1)$}
& \multicolumn{3}{c}{$\lambda_j=U(0.1,0.5)$} & \multicolumn{3}{c}{$\lambda_j=U(0.5,0.9)$} \\
\cmidrule(lr{\tabcolsep}){2-4}\cmidrule(lr{\tabcolsep}){5-7}
\cmidrule(lr{\tabcolsep}){8-10}\cmidrule(l){11-13}
\addlinespace[3pt]
& \multicolumn{12}{c}{Planning Horizon (Years)}\\ \cmidrule(l){2-13}
& 10& 20& 30& 10& 20& 30& 10& 20& 30& 10& 20& 30\\ \midrule
1& I& I& A& I& A& I& A& I& A& A& I& A\\
2& I& I& A& I& I& A& I& I& A& A& A& A\\
3& I& I& I& I& A& A& A& A& I& I& I& I\\
4& I& I& I& I& A& A& I& A& I& I& A& I\\
5& I& P& P& I& I& P& A& I& P& P& P& A\\
6& I& A& A& I& I& A& A& I& A& I& A& I\\ \bottomrule
\end{tabularx}
\end{table*}
\end{document}
utthesis.clsfrom?\label[tab]{results}is supposed to do.tabularxbut you have noXcolumns in your table. What do you expect it to do?\multicolumnentries are longer than the combined widths of the three corresponding columns. For more information see here: Table column widths disproportionate due to multicolumn cell being too long. Probably you could introduce linebreaks in the column headers to circumvent this problem. I'd also recommend that you take a look at the horizontal lines from thebooktabspackage.@{\extracolsep{\fill}}doesn't belong in the picture at all. You should be using suitably modified versions of theXcolumn type instead.