Network discovery failing to run

Hi, have an issue where network discovery is failing to run on a particular SCOM 2012 R2 (UR2) management group.

there's only one management server in the group and I can configure network discovery as normal, run it and the job status shows success. However no devices are discovered, and the last discovered/pending/run time columns all remain blank.

There is nothing at all in the Operations Manager\Network Discovery view under monitoring, and nothing in operations manager log from the OpsMgr Network Discovery source... also I can't see any SNMP requests leaving the management server.

I've tried importing the network device discovery MP again from the SCOM R2 ISO, and I'm scratching my head on any next steps to try short of raising a PSS call. Any ideas appreciated.

thanks