We recently tried to add a 3750x switch to our small group of switches. Up until now, every switch we've added has been visible and configurable via network assistant. I've made sure the trunk vlans are the same, and the switch ip address is within the same group ofclass c address'. From network assistant running on a machine within one of the permissible vlans, the new switch shows as a neighbor, but not correct in the topology. From a copy of network assistant running on a different vlan, the switch is not visible within network assistant. I can telnet to the switch (only from behind the firewall..) so the switch is accessible, it just does not show properly or at all from within network assistant. Doing something stupid I'm sure, just not sure what.
The issue might be as simple as the 3750X isn't supported by Network Assistant. The latter was aimed as the SMB market while the 3750X is an "Enterprise" level (which sometimes presumes you have "Enterprise" level engineers and/or tools).
Ok, is the problematic switch the same exact model as one of your other and running exactly the same IOS version and license?
Good to hear you found the issue and you posted that too!
If possible, see if you can mark your last posting as "solved".