My best guess is that another plugin, or possibly your theme, has some conflicting CSS for the element, or one of the parent container elements of the next link.
Use the “Inspect Element” feature of your browser to look at that element and any of the parent containers, and you should be able to see the CSS that is being applied and where it is coming from. Then, you may need to add your own custom CSS to override the CSS you find for that element, possibly using the !important tag to make sure it can’t be overridden by other CSS.