one of menu item right align

what theme are you using?

can you provide a live link to your site to illustrate the menu?

http://codex.wordpress.org/Forum_Welcome#Include_as_much_information_as_possible

try to add via a custom CSS plugin or child theme:

#menu-main { width: 100%; }
#menu-main #menu-item-665 { float: right; }

if that dos not work, please ask in https://wordpress.org/support/theme/frontier/