mail-sources - how to use?

[Richard Riley]

My understanding from the manual is that gnus should read from the
sources specified in mail-sources and then move the mail found there to
mail-source-directory. (Of course you dont have to use this
functionality and can merely treat using maildirs and select methods for
example).

So if I have :-

,----
|   (setq mail-sources
|         '((file :path "/var/spool/mail/rgr")))
`----

I understand from the info file that that is sufficient for gnus to suck
out the system mail from my account's spool and then by setting

,----
|  (setq mail-source-directory "~/.Maildir/.riley/INBOX/new")
`----

gnus should store this new mail in my maildir which I then access using

,----
|   (add-to-list 'gnus-secondary-select-methods
|                '(nnmaildir "riley" (directory "~/.Maildir/.riley")))
`----

Gnus, however, is not pulling the mail from var/spool/rgr.

What am I missing or what am I misunderstanding?