The task scheduler does a user impersonation, not a full desktop logon. If you are referencing a mapped network drive letter, it may not be available. Use the UNC path, \\ServerName\ShareName\FolderName.
In the scheduled task, set it to execute program cmd.exe. In the arguments field set it to run the .bat file like this.
/c C:\YourScriptsFolder\YourBatFileName.bat 1>C:\YourScriptsLogFolder\YourBatFileName.log 2>&1
In your .bat file, add DIR commands to both the source and destination folders to verify that the .bat file can access your network folders.
Review the .log file and look for errors.