Template localization does not always work, why?

Question

Internationalization and localization

I am localizing the template into Russian.
Through PoEdit I created a localization file, added to functions.php

add_action( 'after_setup_theme', 'my_theme_setup' );
function my_theme_setup() {
  load_theme_textdomain( 'themename', get_template_directory() . '/lang' );
}

If you change the language of the site in the settings to Russian, then everything works, but not always, a strange situation I no longer know where to dig.
I output the text like this, for example Text from some template files is stupidly not translated, why can this be?
__( 'Knowledge Base, 'themename' )