WordPress has a hook system that provides a way for plugins to insert markup into a page – but a theme must implement these hooks in order for this to be possible.
If either of these function calls is missing from your theme, UberMenu can’t include its styles and scripts.
Identifying the issue
Check the source of your page – if the script ubermenu.min.js (or ubermenu.dev.js if you’re in developer mode) is missing, it’s likely your theme doesn’t call wp_footer.
Alternatively, jQuery Enhancements may have been disabled (this only applies to UberMenu 2).
Make sure that your header.php includes the wp_head() function, and the footer.php includes the wp_footer() function.