Hi Vega1, welkcome to the forum!
With the information given, there is no real solution to tell.
- How is your download task setup?
- Are the files from other dates still available on the SFTP server?
What you can do though is using the FTP explorer in the Tools section and open the SFTP connection.
You can see all the files on the SFTP server and you can download files to your local system.
You might find the missing files there.
Uses Visualcron since 2006.