I had similar issue with all readings first not updating and then disappearing after a while — and readings updated only by opening xfce4-sensors-plugin settings. Solved by two things:
1) Removed xfce4-sensors-plugin from panel settings list of items in panel (I don't know exact names in English, because I use Finnish).
2) From directory ~/.config/xfce4/panel removed files .xfce4-sensors-plugin-*.rc and xfce4-sensors-plugin-*.rc (notice the dot in front of the first one, I had one of those configs).
Then just readded the sensors plugin to the panel and did all settings again.
I had similar issue with all readings first not updating and then disappearing after a while — and readings updated only by opening xfce4-sensors- plugin settings. Solved by two things: plugin from panel settings list of items in panel (I don't know exact names in English, because I use Finnish). xfce4/panel removed files .xfce4- sensors- plugin- *.rc and xfce4-sensors- plugin- *.rc (notice the dot in front of the first one, I had one of those configs).
1) Removed xfce4-sensors-
2) From directory ~/.config/
Then just readded the sensors plugin to the panel and did all settings again.