Ensure that the node in question has a check mark in a green circle which signifies that the node is managed. See What is the difference between a "managed" and "un-managed" node? for more information.
If the node appears in grey, its status is unknown. The Connection Manager (CM) has lost connectivity between the node and the Workgroup Server (WGS) and therefore no events or changes are being registered with the WGS for the node.