Nagios check-host-alive command not always reporting accurat

I have a nagios box running that is checking 19 different hosts both for http and a basic check-host-alive. most of the systems are reporting accurately but I have two that are saying the host is down, yet when i ping them they reply just fine. In fact I can even run the exact command that nagios runs from the command line and it comes back with an OK status. Here is the command that is run on the for check-host-alive:
/usr/lib/nagios/plugins/check_ping -H $HOSTADDRESS$ -w 5000,100% -c 5000,100% -p 1

Any Ideas why when nagios runs the command it returns a non-OK status but when I run it from the command line it returns OK?

Take note of the “Last Check” field in the status map. Nagios doesn’t check hosts, in version 1.2 or bleow. It checks services only. If the service fails, it will then and only then check the host with the “check_host” command. Try disabling checks of the host with the “disable checks of this host” link and then wait till the page shows "disabled. Then enable the checks of the host, and notice the “Last check” date is now current.
I repeat, hosts are not normally checked.
Edited Thu Feb 03 2005, 04:55PM ]

hi. I have a nagios v.2.0b3 and the same problem. check-host-alive returns ok status, but nagios shows host down. when i submit passive check through web interface, the host status changes to ok, but just for a moment. could you help me, please?

Paste the output shown in the cgi for this check. Run the command by hand after you have su - nagios first. Show us the output of that command.