When you say you “installed the tools” for the zstick, do you mean the zensys utilities? If so, you should at least have a node topology map. So you’d start by seeing what repeaters the troubled devices are trying to use, if there are any zwave repair error messages associated with the repeater, and in particular if they’re all using the same repeater.
The most common reasons for a cluster of device to fail all at once while other devices continue to work are:
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a common repeater failed
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local interference from a new source, which for zwave is most often a baby monitor or cordless phone. (For zigbee, it’s almost always boosted WiFi)