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
"But I strongly suggest using C-u RET in order to see all the mails instead of unread mails. After C-u RET, you will be asked how many mails to display. If you never delete or move the mail in inbox, the number you provided is usually the exact number of mails to be displayed. If you do delete or move mails in inbox but Gnus is still using the cached data of inbox, you need provide a bigger number instead. Another solution is to delete files .newsrc.eld or .newsrc-dribble and restart Gnus."