SNMP only working for SOME monitors

Im working on setting up all our Cisco switches, routers and access points to use SNMP. I have been successful on about 10 and having problems on 4 ... and I have 40 more to configure.

The problem that Im having is that I configure snmp on the switch, and then set up the monitors in opManager, test them and one, say the CP Utilization will work and then the Memory Utilzation wont work... however if I come back to the snapshot an while later they will work... it seems very intermitent.

Any suggestions? Am I needing to adjust some timeout or something.

I am using a name for my SNMP community does that matter ? Do they have to be public or private or can I call them anything ( Example mine is Tigers for RW )

Thanks in advance.