what kind of server are you doing this on? shared? VPS? Dedicated? DreamCompute?
Using the dreamhost panel, have you ‘filtered’ an email address to forward to a shell account? What filter did you use? if you want 100% certainty all emails would get forwarded (and because I know the dreamhost filters can be flakey for wildcard usage) the best way I’ve found is to use negative logic for the filter, that is “Subject” “Does not contain” “kldjfkdjieoewjfdsklsjk;LDKIE4OWFJ” (i.e. a garbage string that would be very unlikely to occur).
the wiki page on this seems to have been been deleted, but you also need a .forward.postfix file for the shell user to pipe the incoming mail to a script, google for more info since dreamhost chose to delete the wiki page, its standard *nix stuff. Here is were the wiki page was: http://wiki.dreamhost.com/Email_to_Script I didn’t find a replacement.
Lastly, you say you don’t get some emails? Is there a pattern? Like a certain domain?