Hi lschierer,
I strongly suspect it is because your LibraryThing catalogue is not public:
LibraryThing - Settings - Account Settings:
LibraryThing
Match can only work with public catalogues, but it will not retrieve "private" books.
Unfortunately there is nothing I can do, as this is a fundamental part of LibraryThing security/privacy. Let me know if there is anything else I can assist you with.