I'm not able to share the URL, but provide an example below.

I'm working with HTML5 Video. So the <video> element. This has nothing to do
with Flash. I'm getting inconsistent results. Sometimes the poster attribute
of the video element gets downloaded and the path adjusted in the HTML, but
the <source> videos do not get downloaded. Other times the poster image does
not get downloaded, but the video sources do.

Example #1
Here's an example from a site I can share. If you crawl just this page you get
the poster image, but not the source videos:
<http://praegnanz.de/html5video/p_mediaelementjs.php>

(webhttrack -q -%i -w <http://praegnanz.de/html5video/p_mediaelementjs.php> -O
"/home/jnronall/websites/videosws" -n -%P -N0 -s2 -p7 -D -a -K0 -c4 -%k -r1
-A25000 -F "Mozilla/4.5 (compatible; HTTrack 3.0x; Windows 98)" -%F "<!--
Mirrored from %s%s by HTTrack Website Copier/3.x [XR&CO'2008], %s -->" +*.png
+*.gif +*.jpg +*.css +*.js -ad.doubleclick.net/* -%s -%u )

I should be getting the video sources as well here:
<http://praegnanz.de/html5video/player/video_SD.mp4>
<http://praegnanz.de/html5video/player/video_SD.webm>
And the track source too:
<http://praegnanz.de/html5video/player/mediaelement/mediaelement.srt>


Example #2
In my case this HTML:
<video poster="/videos/getting_a_book.png" controls preload="none">
  <source src="/videos/getting_a_book.mp4">
  <source src="/videos/getting_a_book.webm">
</video>

Gets converted to:
<video poster="/videos/getting_a_book.png" controls="" preload="none">
  <source src="videos/getting_a_book.mp4">
  <source src="videos/getting_a_book.webm">
</video>

The mp4 and webm have been downloaded and are playable offline, but the poster
image is never downloaded.

Please let me know if there is more information I can provide to help solve
this issue.