Many thanks @dlmpryor . From that linked thread …
So on the basis that the inbound MQTT messages are not being shown as Inputs within my emonPi, I will take it that the messages are not arriving. They used to, but for some reason stopped, so I will have to investigate why.
A shell script on the other RPi has this content
#!/bin/bash -x
export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/local/games:/usr/games
sleep 20
rtl_433 -F json -M utc -R 43 | mosquitto_pub -t home/rtl_433 -l -h 192.168.2.50 -p 1883 -u emonpi -P emonpimqtt2016
But there is no home/rtl topic within my emonCMS. And looking at the emonHub log file, I see no reference to the word rtl either. So perhaps it is not being generated on the other RPi.