SMP 7.1 SP2 - Inventory for Network Devices - No data if group is scanned

One of our customers imported SNMP MIB files in his Altiris 7.1 SP2 environment which should collect data from network switches. If the customer is scanning one device, all needed information is collected. But if he scans a group (OU) of devices, there is no data.

Customer already tried several groups and created new ones. But no data is collected when an inventory for network devices is triggered on a group. All devices are classified, the discovery scan has run correctly.

Any help would be appreciated.