snmp v2 not responding on Cisco 3750

I am setting up network monitoring.  I have approximately six switches out of one hundred that are not responding to snmp v2c queries.  At first I thought it was an ios version issues, as all of the switches are running 12.2(25) software, HOWEVER, i have two WS-C3750V2-24PS-S   that have EXACTLY the same version of ios and EXACTLY the same configs(with the exception of different vlans), same snmp-server communities, switches are on same subnet - one is working and one is not.  From my workstation with two different snmp tools, one responds to snmp queries, one does not.  I've tried restarting snmp by clearing all the entries, even rebooting the switches with no luck.  HELP.

It turns out, it does have something to do with software version.  Even though the non functioning snmp devices, in many cases, have the same version of software as one that are working, I downloaded the latest version 12 IOS for the devices, upgraded those that are not working, rebooted, and now they WS-C3750X-24P-S   are fine.