DTM client heartbeat not being sent/received

I have a DTM (1.0.c) arrangement set up for cluster testing. I have eight
clients of which only two are providing heartbeats to the DTM controller.
Two days ago, all eight were providing heartbeats. Because of the lack of
heartbeat, the affected clients go to "debug" mode. My clients are set up
identically: How is it that only two are poviding heartbeats? Is there a way
of forcing the heartbeat?

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