Without actually looking at the CSS on the front end of the site, it's not possible to tell what's happening from an image. The left side is clearly the view's build screen, but what are we looking at with the mobile display? Is that the same view build screen shown in a mobile environment or is that a front end preview?

The design screen isn't really meant to work with mobile, but the previews are and I've never seen Formidable's grid fail to be responsive. Formidable's CSS works. There are times when you may want to supplement or override Formidable's CSS. It doesn't appear to be the case here though. It looks like you're working "out of the box".

One thing it could be is that something is interfering with Formidable's CSS like a page builder or theme, but I haven't seen that either. The only thing I can suggest is to post a link to the page displaying the view on the front end and perhaps a community volunteer will be generous with their time and take a look.