Whenever you insert [flexi-gallery] or [flexi-primary] shortcode it may happen that the gallery is not displayed full width.
This is totally depend on the theme you are using. Most of the theme has full width option but say may not have.

To apply full width, you must edit the page where shortcode is present. It will open the page in admin dashboard where you can change your page attributes.

Look at the right side of the page you will see the section named Page Attributes
Under Title change to Full Width Template if available and update the page.
Check image below for reference.