When a slave is not able to connect to mysql on the master with the ispcsrv* user and password from it's config.inc.php file, then the lave can not fetch changes and therefore the changes must stay on master until you fixed that. So the steps to solve your problem are to find out why the slave is unable to connect to master. The most common reason are inconsistencies in the /etc/hosts file between master and slave, followed by closed mysql ports in a firewall and that mysql is not listening on the external network interface. But if you have other slaves that work, then mysql must listen on the external interface. To test the connection, login to the slave and try to connect with the mysql command from slave to master by using the login details (hostname, username and password) listed under dbmaster from the file /usr/local/ispconfig/server/lib/config.inc.php on the slave).