Varying results from scans

I am using Nessus 3.0.1 on Red Hat Linux to scan multiple hosts
on a network. The scan is initiated by a shell script running
NessusClient 1.0.0.RC4 (not via GUI). I am getting different
results during different runs on the same hosts. In particular
there are 2 NT 4.0 based web servers which have OpenSSL security
holes. If I run a scan with only a single target host, it seems
to always find (and report) the vulnerability. When I run the
scan with a target list specified in a file - about 10 targets -
it sometimes misses the same vulnerability. I ran a test with a
couple of hosts deleted from the target list and it found the
hole in one of the 2 hosts that are known to have this hole.

How do I go about debugging this situation? In looking at the
message log on the server side I noticed the following. It said
"Not launching openssl_denial.nasl against xx.xx.xx.197 none of
the required tcp ports are open (this is not an error)" for the
host where it did not find the hole. But it said "launching
openssl_denial.nasl against xx.xx.xx.198 [1060]" for the similar
host where it found the hole.


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