They may have been automatically deleted.

Your email client may have deleted them from the server.

If you connect to KUMail with an email client via POP3, some client software automatically will delete your messages from the Gmail server just after (or certain days after) it download them. Check the related settings in your client software.

[Microsoft Outlook] Open File > Info > Account Settings... and double click on your email address. Click More Settings, open the Advanced tab, and make sure the box next to Leave a copy of all messages on the server is checked.

[Mozilla Thunderbird] Open Tool > Account Settings. On the left side of the list, click Server Settings. Make sure the box next to Leave messages on server is checked.