You've created one or more Subscriptions to subscribe to messages from MessageMaster. However, you've stopped receiving the messages you've subscribed to.
Check the IncomingMessages directory on the MessageMaster Server (shared as MessageMasterQueue$). If a large number of files has accumulated, you are likely experiencing a backlog. If there is a backlog, see "The IncomingMessages Queue is backlogged", referenced below.
If there's no backlog, then there are no messages, and that's why you're not getting any. Ensure that your client machines have MessageMaster Client installed properly, and that they are pointing to the correct path (\\MMServerName\MessageMasterQueue$) to send their messages to. This setting is stored in the registry on your clients. See "Registry Entries for MessageMaster Client", referenced below.
It may also be possible that your clients are experiencing a problem. Check a few of your client machines, and ensure that the "ObjSoft MessageMaster Relay" service is not having trouble starting. If you are also using EventMaster, ensure that the "ObjSoft EventMaster" service is not having trouble starting. Check the Application event log for any errors from these services.