Removing Slug on Custom Post Type

I created a custom post type, and after installing your plugin, I successfully removed the middle part of mydomain.com/partIdontwant/nameofpost. I’ve now created a new custom post type, but I cannot remove the middle part of the URL. I’m seeing mydomain.com/newCustomPostType/nameOfPost, in this case I want to remove newCustomPostType. Why is it working for the first custom post type but not the second post type? thanks in advance.