A problem after moving mail in Gnus

[Tang Yan]

I am just using Gnus for a few days.
Here comes another newbie question.

I use nnml as my mail backend and split mails into different groups.
But after moving a mail from one group to the other, I cannot find the
mail I have moved and the number of mails in that group is incorrect.

What i have done is as follows:

*Group* buffer:
-----------------------
12: gnus.ding
2: mail.private
-----------------------
For some reason, I have to move one mail from mail.private group to
gnus.ding.

I enter the mail.private group, select that mail and press `B m', and
input the name `gnus.ding'. Then the mail is moved to group 'gnus.ding'
and the old line in `mail.private' becomes highlight indicating that
mail is missing.

Then I come back to *Group* buffer, something strange happens:
------------------------
12: gnus.ding
1: mail.private
------------------------
Why the mail number of gnus.ding is still 12, not 13? And I also press
`^' in the group buffer and enter the virtual server. That are still 12
mails, no more and no less.

Then I go to see the ~/mail/gnus.ding directory, finding there are a
mail numbered 13, but how can i read it in gnus?

I guess there is a command like `updating the mail count number', but I
have read the info and find nothing.

Where is my problem? Thank you!