This is a known issue in LibreOffice that was fixed in version 5.3.0. Before this fix when an instance of LibreOffice is running --convert-to or any other command would silently terminate without doing anything.
If your version is older than that you could consider the solution suggested by Frank Yin here. That is close all open LibreOffice documents before running the conversion.
If you cannot close all open LibreOffice documents to run the conversion, or if you find it inconvenient, you could try a solution suggested in the comments to the original bug.
If you have LibreOffice version before 4.5 run
soffice -env:UserInstallation=file:///$HOME/.libreoffice-headless/ \
--headless --convert-to pdf test.docx
If you are running LibreOffice version between 4.5 and 5.3 run
soffice -env:UserInstallation=file:///$HOME/.libreoffice-headless/ \
--convert-to pdf test.docx
Here $HOME/.libreoffice-headless
is a new profile directory different from your main LibreOffice profile.