Lost emails in Gmail

[Thorsten]

Hi List, 

as a long term gmail user, I switched from the gmail-webinterface to
gnus a few month ago. Now, for the second time, I noticed that some
conversations that I remember exactly and wanted to continue, are
lost. I couldn' find them in gnus anymore, and using the web-interface
did not help - the emails seems to have disappeared from the server.
That never happened to me before using gnus (imap) with gmail.  Is that
a known problem related to gnus? Does it happen to others too? Why does
that happen, and is there anything that can be done against it?

Cheers
Thorsten