If you are experiencing issues with your local mail aliases, make sure there is no syntax error in /etc/aliases file and have a look at the mail log file :
/var/log/mail.log
You most likely will encounter a message like this one :
postfix/local[15083]: warning: database /etc/aliases.db is older than source file /etc/aliases
In that case, simply run the command to rebuild the aliases database (as root or using sudo) :
newaliases