If group is created in NPC containing just poll instances the poll instances may fail to sync down to NV.
If a group is created in NPC containing some devices and then some poll instances from another device so that only those poll instances are part of the group and not the device itself the group gets created in NV with the devices but not the poll instances.
In the NV Console select the group then on the right select Membership. Add a rule to select the missing poll instances and then click Run. This correctly added the devices to the group.