Well, there are so many things that could go wrong when polling via SNMP network equipments.
In short, you should check
-the SNMP version used
-the ACL on the switchs (is your server authorized ?)
-the routes (does your server know the route to the switch ? and does the switch know the route back ?)
Also, check the logs on the switches: do they see the SNMP request ?
Sorry I can't help you further, but as you can see, there are quite a lot of things that can be wrong, and almost all are outside nagios (check the command definitions, just in case ...)