Cause the pattern is only set to detect one directory.
How many more directories do you need? Are you talking about that $2 that's supposed to go in 'Author=' ? If so, try this:
RewriteRule ([a-zA-Z])/([a-zA-Z])$ mypage.php?Source=$1&Author=$2 [L]
EDIT: My apologies. There was an error in my pattern, though 'tis fixed now.