Overview of the issue
In some rare cases after updating from an older version, translations are not visible after you translate a string. This happens if the update and migration steps get stuck.
Add the following code to your wp-config.php:
define( 'WPML_ST_SYNC_TRANSLATION_FILES', true );
In the database, go to the options table (usually wp_options) and remove the key with the _transient_wpml_st_upgrade_in_progress name.