missing gnus-unseen-mark

[Dan Davison <davison@stats.ox.ac.uk>]

I'm using dovecot with nnimap and local maildirs.

I'd like a mark to remain by messages in the summary buffer until I have
actually read them (presumably that would be gnus-unseen-mark).  When I
enter a group, new messages have the 'N' mark (gnus-recent-mark) in the
summary buffer. If I leave that group (without reading anything) and
then re-enter, the 'N' mark has disappeared and is (often) not replaced
by any mark. Actually I do sometimes get the '.' mark which aIui is
gnus-unseen-mark, but by no means always.

What should I do in order to reliably get a persistent gnus-unseen-mark
next to messages until I have read them?

Thanks a lot,

Dan

Here's one possibly relevant variable setting:

(setq gnus-summary-line-format
      (concat
       "%0{%U%R%z%}"
       "%3{│%}" "%1{%d%}" "%3{│%}" ;; date
       "  "
       "%4{%-20,20f%}"               ;; name
       "  "
       "%3{│%}"
       " "
       "%1{%B%}"
       "%s\n"))