The extension doesn't remember the records it processed. It will process any records where the output field is empty and skip any record that's filled already.
To reprocess a record again, you have to: Clear the value in the output cell to be empty.
If you're using the URL to attachment extension, you'll notice that -by default- it's not redownloading the updated file from your URL. If you're trying to re-upload the attachments when the source URL changes, you'd need to make sure the output field is empty.