There should be a setting in the plugin that allows you to set it at Full Width. If you have a fixed width added in the settings then it won’t be 100% full width/flexible. So double check all of your settings to make sure you have it set to full width and not a fixed width.
At first this plugin can seem overwhelming since there are a lot of settings to go through at first, but the option should definitely be there.
Also, if you have a link that would be good so we can see what it looks like.