From the command line of your Zenoss server, can you please try this command :
snmpwalk -v 2c -c your_community_name IP_of_device_not_working system
and post here the results. You must use the real SNMP community name as it is configured in your switch and the IP address of one of the switches that gives you the SNMP error.
Are you able to spot any difference (device model, firmware version, interface type etc.) between the switches that are fine and those that report errors ? What is the version of SNMP you are using ?