I have installed Loco Translate in a local installation in order to investigate the issue.
After syncing a lot of new untranslated strings appeared in Loco Translate.
I translated them and got the new .po file.
I have uploaded in my live installation and everything works properly now.
The problem persists in missing strings in your .pot file.
Please check attachment.
The specific one as all the other untranslated strings are missing from your .pot. (css class shall be included in string)
So if you try to translate manually there is no chance of translating them.