Check the following:
For the system(s) not working, make sure there is a /var/yp/nicknames file. Also, make sure that this file contains this entry: aliases mail.aliases
On one of the systems not working, execute the following:
You will probably get this message: no such map in servers domain. Do a ypwhich to see which NIS server the system is bound to. Next, go to that server and verify that the mail.aliases map is missing from /var/yp/domainname. This map must either be created or copied over from one of the NIS servers that contains the map.