When a visitor accesses your site, she is assigned one of the possible alternatives for each of the running experiments. From that moment on, whenever she accesses a tested page, post, or element in general, she'll see the same version (otherwise, she may detect she's being A/B tested). This information is stored in a cookie, so if you want to check whether all alternatives are being returned, you'll have to clean the cookies between different tests.
Nonetheless, there are a couple of exception to the previous statement. If you always see the original version, please keep in mind that:
The plugin does not test admin users. If you're logged-in and you try to access the tested page, you'll always see the original version.
If you want to get assigned one of the alternatives, consider (a) logging out, (b) accessing your site from a different browser, or (c) access it using private browsing mode.
The plugin is compatible with cache systems, but it does not automatically clean them. Our plugin is able to "bypass" cache systems and return the appropriate alternative to each user, preventing all users from seeing the same cached version. In order to do that, all pages in your website have to include our tracking scripts, which are automatically included by the plugin itself. However, if your site is running behind a cache, it is possible that your users are being served a cached version of your site that does not include our scripts.
In order to fix the issue, please make sure you clean the cache right after you activate our plugin (some cache systems for WordPress do this automatically for you when a new post is published, a theme is changed, or a plugin is installed).