Display all messages, including read ones

Hello,

When there is an unread message in a long thread, the group buffer will
indicate that there is an unread message. When I press <ENTER> to visit
that group, the cursor will show that message. But often, in a long
thread, gnus will not show the whole thread, even thoug the parameters

    (display . all)

is configured on the group: only the root, and the last few messages
appear. Even when pressing "A T" (gnus-summary-refer-thread) I still do
not get some of the old, unexpired read messages. They do not show up
even when I visit the root and press "T s" (gnus-summary-show-thread) I
still do not get the thread.

What I expected was that I would be able to see all the message in the
thread when I visit the unread message.

However, if in the group buffer I press C-u <ENTER> (which prompts me to
enter the number of messages to display), only then I see all the
messages in thread, which also includes expired ones.

I am not clear why gnus behaves this way. It would be great if someone
can help me root cause this issue. Possibly relevant configs I have:

    gnus-show-threads: t
    gnus-refer-thread-limit: 500

Thanks,
Husain