First of all, make sure that you are following String Translation procedure :
1. Go to WPML > Theme and Plugins Localization 2. Scan the theme (or plugin the strings come from) for strings
3. After the scan is finished, go to WPML > String Translation
4. There you should try to find the string - and translate it (make sure that the translation is marked as complete and saved)
5. If the string is not in the String Translation, you can try to add this string from "Translate texts in admin screens" (you can find it on the bottom of String Translation page)
6. When you get in the Admin Screens* and find the strings, then mark them and go to the bottom of that page and click "Apply"
7. After you click "Apply" wait for the confirmation message
* Admin Screens explanation : The concept of Texts in Admin Screens is to add the strings that are stored in the database, to be precise in wp_options table. So if the theme or plugin stores some strings inside wp_options table, you will be able to translate this string. You should find option_name and expand it, then add this string to ST.