Gnus sometimes reports new messages but not showing them on IMAP server

Hi,

I'm having some issues that Gnus sometimes reports that there are new
messages on an IMAP server but it doesn't show them.  In my case it's
using Gmail IMAP, and if I check using other mail clients
(e.g. Thunderbird) or directly check on Gmail website I can seem them.
I'm not sure whether this issue is specific to IMAP either.  As I'm not
sure how to get more info, I didn't use report bug yet, but would like
to ask around and see whether this is a known issue, or otherwise I can
get some suggestions to get more debug info before I formally file a
bug.

Any suggestions are welcome!  TIA!

-- 
Xiyue Deng