There are two ways to make an embedded video responsive:

1. (preferred) Put a full (not minified) link to the video in a text block. In order for this to work, the link must be the only thing on its line and must preceded and followed by an empty line.

2. If point 1. doesn't work for some reason, do the exact same thing but wrap the link in an [embed] shortcode:

[embed] [/embed]