Mobile responsiveness issue

Hello there!

Most of the pages on my website are horizontally divided into two parts (with code [one_half][/one_half][one_half_last][/one_half_last]). One part is usually text, the second part image or video. Please see for yourself on http://www.meetmybarcelona.com/nl (please make sure to put nl in your url, the other pages are under construction)

My theme is mobile responsive. And it works fine when the screen width is very small: then the two parts show up one below the other.

But when the screen width is not small enough (for example, when i hold my mobile phone horizontally), the two parts stay next to each other, although the video and the images become way too small to be userfriendly.

Ideally, I would like the two parts to show up below each other much sooner. Is there some solution to this? Could i put some kind of code in that makes the two parts go under each other when the screen size decreases? For example set a minimum width to the images and video?

Sorry for my non-technical description here, I am kind of a wordpress newbie.

Thank you for your help!