Indentation not working for some paragraphs

Some paragraphs in my document do not indent, what ever I try. I have solved some of them using the \parindent and \usepackage{indentfirst} but these specific ones do not seem to work. In those specific paragraphs, I include some math environments. Following each paragraph is a math environment on its own line. Can someone tell me how to fix this?

Here is the code and subsequent pdf view:

The added code at the beginning of the document:

• Depending on your \documentclass, the indentation may be set to zero by default. Add, for example, \parindent=15pt\relax to your preamble and see what happens upon recompilation. – Steven B. Segletes Jun 21 '17 at 11:55
• Unrelated, but you shouldn't use $$...$$ in LaTeX (tex.stackexchange.com/q/503). For single line equations, use $...$, for multiline equations without alignment (like your first two equation blocks, add \usepackage{amsmath} if you haven't already, and use \begin{gather*} first line \\ second line \\ third line \end{gather*}. – Torbjørn T. Jun 21 '17 at 12:00
• never use a blank line before display math (and don't use  for display math in latex) you appear to have parindent set to 0pt. but as you have shown no code it is hard to be sure. – David Carlisle Jun 21 '17 at 12:07
• Unfortunately @StevenBSegletes, the \parindent=15pt\relax did not change a thing. I had set \setlength{\parindent}{6ex} – Marco Froelich Jun 23 '17 at 20:13
• It is better to post code itself, rather than images of code. That way, those who would otherwise help you can copy/paste the code. With an image, that is not possible. – Steven B. Segletes Jun 23 '17 at 20:18