I know it's late, I have the same issue with an old custom theme, just added to boostrap.css:
.embed-responsive {
position: relative;
display: block;
height: 0;
padding: 0;
overflow: hidden;
}
.embed-responsive .embed-responsive-item,
.embed-responsive iframe,
.embed-responsive embed,
.embed-responsive object,
.embed-responsive video {
position: absolute;
top: 0;
bottom: 0;
left: 0;
width: 100%;
height: 100%;
border: 0;
}
.embed-responsive-16by9 {
padding-bottom: 56.25%;
}
.embed-responsive-4by3 {
padding-bottom: 75%;
}
And for the video:
<div class="embed-responsive embed-responsive-16by9" >
<iframe class="embed-responsive-item" src="https://www.youtube.com/embed/jVIxe3YLNs8"></iframe>
</div>