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More than 100 target segments left empty after 'update file'

Hello, I am a new user of Trados Studio (2019) and have worked on my first projects with this software.

I had some bad quality files that I prepared (PDF to Word) before I imported them in Studio. And of course, with the largest file I had to work on today, I had to change something in the prepared Word document. After I did that, I updated the file in Trados and saw that 100 target segments were left empty after the update.

To be honest that scares me, but since I am a new user, I might have done something wrong or I am missing an option that I should have set. And in that case, is there anything I can do to get those segments filled again? I tried pre-translate, but that didn't seem to work. 

In case I did something wrong, please try to enlighten me.

Next time I will make an update of the sdlxliff of the file that I am updating, but that shouldn't be necessary every time a source file is updated.

Thank you,
Michael