There’s two possible issues, both caused by your theme (not this plugin).
1. You don’t have column classes in your CSS. It should be included in all Genesis child themes, but there’s a chance it might not be in your theme. Open your style.css file and search for one-half. If nothing comes up, you’re missing this section: https://gist.github.com/4198285 This seems to be the issue with Andre’s site since all the posts are full width.
2. You have more specific styles that are breaking the CSS. This is most often caused by not using border-box as the box-sizing and then adding padding to the individual entries. If you’re using content-box (the default), when you define a width, the ACTUAL width is your width + padding. With border-box, the actual width is your defined width + padding, so usable width is less. IE: if I say width: 200px, padding-right: 10px; the usable area with content-box is 200px, with border-box is 190px. This is typically the issue if you see your posts showing up small but not actually floating next to each other correctly.
