Non-responsive video

I am starting to work with Museum Core. Works great except for embedded YouTube videos in windows between 1024 and 1200px. The video overlays the sidebar. I hadn’t noticed until I changed from the 9+3 col standard to an 8+4 layout. It is the same with all videos I tried and in all browsers. I’m sure it is a setting issue. Any thoughts on what I need to change? Thanks.