Rule of the house : You should provide us enough detail so we can be able to help you.
What is the version of Zenoss you are using ? What is the switch manufacturer and model ? Do missing graphs show NAN, a value of 0 or they are completely missing (screenshots) ? Anything special in the zenperfsnmp and/or other logs ? Can you still see SNMP information in device overview page (uptime, system name, OS etc.)?