Gnus agent: Baffling offline behaviour without LIST ACTIVE

[mark@ixod.org (Mark T.B. Carroll)]

I have Gnus connect to a few NNTP servers. Each is marked (agent) in the
servers buffer. Everything works fine with downloading articles and looking
at them offline for all except one of the servers. In comparison with the
others, I can't see anything remarkable in its configuration. However, if I
poke around in News/agent I see that its active file is unusual in being
empty.

Indeed, it's the only server that doesn't support LIST ACTIVE. With Gnus
unplugged, I can't use any of its groups. However, here's the really
strange thing:

Gnus agent has downloaded all the articles in those groups. They're there
on disk, it just refuses to access them. Moreover, when plugged Gnus does a
GROUP for each of the relevant groups on that server, so it does have the
information it needs. And, it seems to need to go back to the server for
the articles even though they're already downloaded.

What's going on here? The active files for the other servers don't seem to
contain anything that Gnus wouldn't have got when it did the GROUP commands
instead of LIST ACTIVE on the one server I'm having trouble with. How can I
get it to deal with this server in the same agreeable manner as it does the
others, so that I can read news from it with Gnus unplugged?

(I apologise if this appears multiple times - I seem to have limited
control over Gnus so far! I'm aware of one accidental posting that I
immediately cancelled.)

-- Mark