Yes, some of the string are in theme po file and some are in plugin po file.
Finally found the solution!
some of the element missing translation were into the THEME .po file
that's the problem.
despite having updated theme and plugin to the new 1.2.0, I still have problems with translations, some elements are not translated despite having found them with Loco, others are not even present in the .po file.
Can you give me some more info?