so...IPAM is as good as its access allows it to be. Things to keep in mind:
if it can't ping it, its not there [automagically].
NATs/PATs can be an issue due to this (also...firewalls typically don't allow hairpinning)
Systems with local firewalls enabled have to allow ping from main NPM box.
Sometimes you can use neighbor routers to fill in blanks.
My NPM is "inside" of firewall. I have a bunch of NATs on "outside" for web access, etc... I point that subnet's "Neighbor Scanning" to that router and run a TCL script on it daily to ping sweep that subnet. That makes sure the ARP table has everything and my IPAM is as complete as possible.
IPAM grabs the ARP table and populates/updates itself based on that.