[Tex/LaTex] How to enable ligatures

Question

I just notice that ligatures are not working on my system. For example:

\documentclass{article}
\renewcommand{\rmdefault}{ptm}
\begin{document}
data-flow missing ligature.
\end{document}

The "flow" word does not use the f-l ligature, appearing ugly.

This also happens with the default article font (if I removed the renewcommand):

I am setting with pdflatex.

Do I have to include a particular package?

Answer 1

sorry to say, this is the fl ligature.

here is the output of running tex testfont on ptmr7t (the roman instance of the ptm fonts):