Robocopy not copying files in subdirectories

There's a /S option in robocopy, too. Also a /E that is like /S, but additionally forces empty folders to be copied.

/S : Copy Subfolders.
/E : Copy Subfolders, including Empty Subfolders.

Supposedly, /MIR implies /E…

/MIR : MIRror a directory tree—equivalent to /PURGE plus all subfolders (/E)

…but I have not found this to be empirically true. You should include the /E or /S option explicitly.