I've had this problem for a while and apparently it hasn't been resolved since others are posting about it here as well, but I'd also like to reiterate:
When using the shortcode with a playlist, the playlist does NOT automatically update when videos are added to the playlist via YouTube. Even after 24 hours, the video still won't show up on the WP site.
The only workaround I have, if you can call it that, is to re-create the playlist and use a new playlist ID. This is a terrible solution. The plugin itself should automatically update.
Please fix ASAP. I'll mark this plugin broken until it's fixed. Thank you!