Some of the Overleaf bibliography entries in my Bibliography.bib are not recognized ( I get a question mark in the pdf when I \cite{} )

Question

I am using Overleaf to write my thesis and it has the following structure:

\begin{document}

\makemtgtitle
\pagenumbering{roman}
\include{Abstract}
\tableofcontents 

\include{Introduction}
\include{Related Work}
\include{Background}
\include{Methodology}
\include{Experiments}
\include{Results}
\include{Discussion}
\include{Conclusions}
\appendix
\include{Appendix}
\listoffigures
\listoftables

\bibliographystyle{plain}
\bibliography{Bibliography}

\end{document}

In Bibliography.bib, almost all of the references (around 70) are recognized, but for a few of them I get the error 'bib entry not citation on page X undefined on input line Y'. This an example reference that was referenced correctly:

@inproceedings{moccia2020vision,
  title={Vision-based dynamic virtual fixtures for tools collision avoidance in robotic surgery},
  author={Moccia, Rocco and Iacono, Cristina and Siciliano, Bruno and Ficuciello, Fanny},
  journal={IEEE Robotics and Automation Letters},
  volume={5},
  number={2},
  pages={1650--1655},
  year={2020},
  publisher={IEEE}
}


and one that was not recognized:
@inproceedings{chen2016virtual,
  title={Virtual fixture assistance for needle passing and knot tying},
  author={Chen, Zihan and Malpani, Anand and Chalasani, Preetham and Deguet, Anton and Vedula, S Swaroop and Kazanzides, Peter and Taylor, Russell H},
  booktitle={2016 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)},
  pages={2343--2350},
  year={2016},
  organization={IEEE}
}

I cited these references using \cite{moccia2020vision} and \cite{chen2016virtual}.

I double checked if there is any error in the Bibliography.bib and .tex files.