Columns with field group

Hi, thank you for your wonderful theme, just upgraded from Bootstrap 3.

Completely baffled:

Using field group to create 2 columns on a page.

Field Group Hero = div = col-md-6
- image
- other fields

Field Group Side = div = col-md-6
- video
- other fields

But field groups are just appearing below each other, instead of side by side 🧐

I'm probably still stuck with bootstrap 3 in my head, please would you kindly advise me?

Most appreciated.