I had the same problem with only about 3 computers not showing up as scanned. The fix I found was related to the operating system on the client. The services for "server" "workstation" "Computer Browser" and their dependencies were not working. Basically they (users) could get out on to the network, but we could not get back to them. LS would show them as "Reverse lookup failed" or "The network path was not found" even though we could ping them and they could see out with no issues. The fix was to re-extract the files "c:\windows\system32\SRVSVC.dll" and "c:\windows\system32\drivers\srv.sys" from the xpsp2 cd and reboot. Then they worked fine after that.
Hope this helps someone.
LS 3.2
450 Clients Workstations in 11 states