Synch project mapping rule not applied.

I've a sync project (PowerShell) that uses a virtual attribute to transfer membership data, however when I change data in OneIM the synch engine doesn't trigger an update to the target system. I can see that the member mapping rule (membersOut) is identified but it is not applied. The synch direction, workflow and mapping are all set to TargetSystem. There are no contraints such as Do not overwrite or Conditions for Use.

DEBUG (SystemMappingRule membersOut Job be3d24a3-72bb-439f-ae9c-c9c1b89a0055) : Perform mapping between IAMTestGroup@UNSGroupB[] and IAMTestGroup@Vault@VaultGroup[].
TRACE (SystemMappingRule membersOut Job be3d24a3-72bb-439f-ae9c-c9c1b89a0055) : Options:
TRACE (SystemMappingRule membersOut Job be3d24a3-72bb-439f-ae9c-c9c1b89a0055) :
TRACE (SystemMappingRule membersOut Job be3d24a3-72bb-439f-ae9c-c9c1b89a0055) :
TRACE (SystemMappingRule membersOut Job be3d24a3-72bb-439f-ae9c-c9c1b89a0055) :
DEBUG (SystemMappingRule membersOut Job be3d24a3-72bb-439f-ae9c-c9c1b89a0055) : Mapping not applied.

Does anyone know of any factors that might affect whether the mapping is applied?

Cheers,

Craig