I am using gnus for reading my gmails. But almost always by checking the INBOX of my gnus and comparing to my actual gmail account, i have new (or old) mails which are not shown in gnus. Does anyone have an idea what could be the problem?
Emacs gnus automatically removes emails marked as read, even if read by another email client. It does not remove them however, but only hides them. They can be viewed by entering /o which insert all old articles, you will need to set the scope either select the default value given or insert your own scope.
It is also possible to permanently disable hiding read emails. I don't do it myself but here's a link: https://stackoverflow.com/questions/15703346/show-all-messages-in-gnus-with-display-all