I am experiensing host detection problems when I use the full vulnerability scan against my servers. Especially when it comes to Virtual Servers. From a group of such type of servers hosted in a sinlge server, Foundstone manages to actually scan only one of them...and that was not the host server...but one of the virtual ones...Any suggestions why that could happen? Just to let you know those are Solaris servers. Actually, I have experienced plenty of OS Misidentification and Disability in Host Detection...I don't know if that is the reason but it is a fact.
Thats hard to answer with out looking at the daily log files and a pcap. If I had a support case this is how I would go about finding the issue.
My guess based on lots of experience is that your targets didn't respond but without data to look at its hard to say.