snmp on 3com superstack

I am trying to add snmp based traffic monitoring to a 3Com supersatck 4900 switch.

when adding traffic monitors -> Interface RxTraffic, interface Tx Traffic etc and run test monitor, I get an error message saying no response from device and with possible resons: Snmp not enabled etc.

When I go to the mib browser and use RFC 1213, go to ifInOctets etc and do a get, I get a response with the counts for the traffic. (so I suppose it is not a problem with the passwords and community strings?)

The swich is seen with the blue star and all ports are visible on th Opmanager. The traffic however does not monitored.

I have enabled monitoring with OPManager for a similar switch and it works fine.

What could be the problem?

I have also used the command line to interogate the switch with snmp gets and they work fine as well.

Thanks