Indentation not working only for some paragraphs [duplicate]

Question

My indentation has stopped working after one section of my document. Here is the beginning code:

\documentclass[11pt]{article}  
\usepackage{amsfonts}  
\usepackage{amsmath}  
\usepackage{amssymb}  
\usepackage{lastpage}  
\usepackage[margin=0.85in]{geometry}  
\usepackage{graphicx}  
\usepackage{tabu}  
\usepackage{textcomp,gensymb}  
\usepackage{caption}  
\usepackage{float}  
\usepackage{indentfirst}  
\usepackage{multirow}  
\usepackage{array}  
\usepackage{subcaption}  
\setlength{\parindent}{6.5ex}  
\setlength{\parskip}{1.5em}  

\begin{document}
This is the part when it does work anymore.

\section{blah blah}  
Blah blah blah, $math$, blah blah, like blah:  
\begin{gather*} more math \\ more math \\ more math \end{gather*}  
Blah blah... 

\end{document}

When I start including math environments within the paragraphs and outside, it does not seem to work anymore, what ever I try. I have tried using the \indent, using \setlength{...}, specifying all paragraphs using \par at the end, ending with \\, nothing works.

This is the part when it does work anymore.

\section{blah blah}  
Blah blah blah, $math$, blah blah, like blah:  
\begin{gather*} more math \\ more math \\ more math \end{gather*}  
Blah blah... 

After this section, no indentation works.

Answer 1

I solved it. After the new section, just add the command:

\setlength{\parindent}{...}

Then it works again. This probably gives me some blue errors, but I've learned how to ignore them.

Ex.

\section{blah blah}
\setlength{\parindent}{6.5ex}
blah blah blah (is indented)