Host name Enter a unique host name. Alphanumerics, spaces, dots, dashes and underscores are allowed.
Note: With Zabbix agent running on the host you are configuring, the agent configuration file parameter Hostname must have the same value as the host name entered here. The name in the parameter is needed in the processing of active checks.
Take a look at these links and check if they are of help:
http://blog.app4domain.com/2014/11/fix-zabbix-agent-no-active-checks-on.html
https://www.zabbix.com/forum/zabbix-help/45122-agent-server-active-check-not-working