"Zone on DNS Server is not responding to queries". But the zone is responding

Hi,

This monitor "DNS 2008/R2 Zone Resolution Monitor" is giving us this alert "Zone <Zone name> on DNS Server <Server Name> is not responding to queries" for 2 of our zones, according the Knowledge Base of the monitor this command is issued: nslookup -querytype=ns <zone name> <server>. When I run this command  the zone response to it, so I'm not sure if there is an issue with the monitor.

I'll appreciate any suggestion.