Unable to display citations using "plainnat" style

Question

I noticed that it my document I get a "?" where the citation should be when using

\bibliographystyle{plainnat}

However, the numbers appear when I use

\bibliographystyle{apalike}

I have a preamble.tex file from a template which contains the following code related to bibliography

% *****************************************************************************
% *************************** Bibliography  and References ********************

%\usepackage{cleveref} %Referencing without need to explicitly state fig /table

% Add `custombib' in the document class option to use this section
\ifsetBib % True, Bibliography option is chosen in class options
\else % If custom bibliography style chosen then load bibstyle here

   %\RequirePackage[square, sort, numbers, authoryear]{natbib} % CustomBib
   \RequirePackage[round]{natbib} % CustomBib

% If you would like to use biblatex for your reference management, as opposed to the default     `natbibpackage` pass the option `custombib` in the document class. Comment out the previous     line to make sure you don't load the natbib package. Uncomment the following lines and spec    ify the location of references.bib file

% \RequirePackage[backend=biber, style=numeric-comp, citestyle=numeric, sorting=nty, natbib=    true]{biblatex}
% \bibliography{References/references} %Location of references.bib only for biblatex

\fi

I tried both including and not including "custombib" in the documentclass options but it didn't make a difference. My main file report.tex contains

\input{preamble}

to use preamble.tex. And then the bibliography is called as follows:

\bibliographystyle{plainnat} % use this to have URLs listed in References
\cleardoublepage
\bibliography{References/references} % Path to your References.bib file

I also made sure to delete the .aux, .bib, etc. (all the intermediate) files before recompiling.

I am using vim-latex with the default compilation sequence to generate the PDF.

Any thoughts on why the citations aren't showing up? Thanks!