messages always marked unseen in maildir

[Alfredo Buttari]

Hello,
I am fetching emails from my Gmail imap mailbox with offlineimap to
get them stored on my disk in maildir format. I'd like to use gnus to
read them but I'm facing a couple of  problems

1) first time I load the messages, they all appear as unread even if
they have been read through the Gmail interface (and there is an 'S'
appended to the corresponding filenames in the maildir directories)

2) even when I open/read a new message, it is always marked as unseen
(the 'S' character is not appended to the maildir file corresponding
to that message). Thus, even if the message appears as read on the
gnus interface, it is shows as unread on the Gmail one.
Here is (the relevant part of) my gnus configuration:

(setq gnus-select-method '(nnmaildir "Gmail"
          (directory "~/Mail")
          (directory-files nnheader-directory-files-safe)
          (expire-age never)
          (get-new-mail t)))

can anybody help me with this?
thanks

alfredo