This seems to be one of the most common problems for new user of Nagios. Most all of the time this problem is due to an incorrect command definition for the host check command you specified in the host definition.
Make sure that the host check command in your host definition is configured correctly. The host check command is not a service - it is part of the host definition. Also, the host check command is not executed at regular intervals like a service is. Rather, it is executed only when Nagios finds it necessary to check the status of the host.
Important! Just because you have a service that is monitoring ping statistics for a host does not mean that the actual host status is being checked. The status of a host is only checked when a service check results in a non-OK state or if the host was previously down and a service check results in an OK state.
Some symptoms of incorrect host check commands include:
[list]Hosts incorrectly being listed as DOWN [/list:u]
[list]Hosts incorrectly being listed as UNREACHABLE [/list:u]
[list]Alternating alerts/notifications about host problems and recoveries[/list:u]
Marc, show me your host_definition Directive...