When PigVision is checking if slaughter data has already been downloaded, it is always done using the unique Statement ID which is present on all slaughter data. If you delete the Statement ID, you will delete all information belonging to the download and your data will be available and can be downloaded again.
Enter the menu Progeny – Manage Slaughter data – Slaughterhouse. Here you can delete the deliveries you want to download again. When you do the download again please remember to alter the starting date, only this way you can assure that all data is downloaded.