有时引用有效，有时无效

Question

我正在使用 Elsevier 的模板，遇到了一个有趣的问题。有时引用有效，但有时无效，并显示一个问号，如“[?]”。

我的文件如下所示：

\documentclass[final,3p,times,twocolumn]{elsarticle}
    \usepackage{graphicx}
    \usepackage{amssymb}
    \usepackage[cmex10]{amsmath}
    \interdisplaylinepenalty=2500
    \usepackage{array}
    \newcommand{\PreserveBackslash}[1]{\let\temp=\\#1\let\\=\temp}
    \newcolumntype{C}[1]{>{\PreserveBackslash\centering}p{#1}}
    \newcolumntype{R}[1]{>{\PreserveBackslash\raggedleft}p{#1}}
    \newcolumntype{L}[1]{>{\PreserveBackslash\raggedright}p{#1}}

然后主体是：

\journal{Vehicular Communications}
    \begin{document}
    text goes here...
\cite{ieee2010ieee,etsi2010302}...\cite{resendes2010vehicle}
    ...
    \bibliographystyle{elsarticle-num}
    \bibliography{elsdoc}
    \end{document}

我的 bib 文件是 elsdoc.bib

@article{ieee2010ieee,
      title="\protect{IEEE Standard for Information
Technology--Telecommunications and information exchange between
systems--Local and metropolitan area networks--Specific
requirements--Part 11: Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) specifications Amendment 6: Wireless Access in
Vehicular Environments}",
      author="\protect{IEEE 802.11 Working Group} and others",
      journal="IEEE Std",
      volume="802",
      number="11",
      year="2010"
    }
@article{etsi2010302,
      title={\protect{302 665 v1. 1.1: Intelligent transport systems
(ITS)}},
      author={ETSI, EN},
      journal={Communications architecture},
      year={2010}
    }
@inproceedings{resendes2010vehicle,
      title={Vehicle-to-vehicle and safety pilot},
      author={Resendes, R},
      booktitle={Proceedings of US Department of Transportation Safety
Workshop},
      year={2010}
    }

完整模板是https://ctan.org/tex-archive/macros/latex/contrib/elsarticle