Certain strings may require a specific procedure to be located and translated.
Moreover, the proper method for translating strings greatly depends on how they have been incorporated into the theme/plugin. Some plugins might not even be written in a way where the coded strings are implemented in an "internationalized" way so that is why no multilingual plugin might be able to see the strings out of the box without some extra work upfront.
IMPORTANT: Sometimes, there are even duplicates of the same string and only one of them is the correct one that needs to be translated.
Also, sometimes, simply removing the string (checking the box and removing it) and then re-registering it again by scanning for it (one of the methods below) and re-translating it resolves the issue.
Please attempt both methods and check if either of them resolves the issue.
Method 1:
Navigate to WPML > String Translation, and at the bottom of the page, you will find a checkbox labeled "Look for strings while pages are rendered." You can enable (check) that option, then go to the front end to a page where the string is displayed, and afterward return to WPML > String Translation to search for that string and see if it now appears.
Method 2:
If you can't locate them within WPML > String Translation, they might be included as admin text strings.
