Responsive Video Issue

[shack1nr]

Hi there! I'm not sure if you're able to help, but I'd love it if you could check over my screenshots and see if you've got any ideas of why I'm having a responsive video issue. I tried embedding a video video into a Wordpress site im currently working on and it keeps snapping to smaller sizes when the browser gets smaller instead of staying fluid. When viewing it on a mobile device or at a mobile size in the browser, it snaps to a tiny thumbnail instead of staying the full width of the screen. Any ideas as to how to fix this on my end?

Here are some screenshots of the issue:

Thanks so much! This code is really great--I appreciate it!!

[shack1nr]

Also--It may be easier to see the issue if you click on the images so you can see their actual size. Thanks!