If items preceding \only<X>{\includemovie} have overlay spec strictly higher than X, let's say Y > X, then the movie does not appear. It looks like it thinks the movie should appear on slide Y instead of X.
Minimal test case: https://www.overleaf.com/read/pxvrsdgjscbc
\documentclass[draft]{beamer}
% Yeah, it's obsolete, but media9 is even more so now that Flash is dead
\usepackage{movie15}
% Notes from movie15:
% 1. Options `autopause' and `autoresume' should be set, as this prevents the
% movie from being rewound when passing from one overlay to the next.
%%%%% -> BTW I never got this to work, overlays always reset movies for me, tried in Okular and pympress.
\begin{document}
\begin{frame}[t]
\frametitle{Cool video frame}
Here is a cool movie coming up.
\visible<3->{That was a cool movie, eh?}
% This wouldn't break it, but the overlay spec cannot be after the movie
% \visible<2->{That was a cool movie, eh?}
\only<2>{
\centering
\includemovie[url]{0.8\linewidth}{.45\linewidth}{coolmovie.mp4}}
% This works, but limits us to only specify stuff after \includemovie
% \visible<3->{That was a cool movie, eh?}
\end{frame}
\end{document}
What happens: 1
The expected result can be obtained only by placing \visible<3->{That was a cool movie, eh?} after \includemovie (uncommenting the last one). 2
Cross-referencing issue opened on Beamer Github, as I believe this is a bug: https://github.com/josephwright/beamer/issues/733
