@mikejhathaway did you find a solution please? I have an identical issue. Happening on just one of my devices. 

  @Henryshelley nothing yet, I've opened a case with TPLink, so far the advice has been factory reset and then calibrate, all of which I have already done, will see what they come back with tomorrow! I personally think the device is faulty if 3 others purchased at the same time work just fine!

  @mikejhathaway If it helps, I changed the batteries in the unit that wasn't working, ran a calibration too, and now it's back working again. On the unit itself it was displaying "Lo" which means low battery (so presumably it had stopped opening and closing the valve correctly). But normally it says there is a low battery in the app too (but this didn't happen). 

  @Wayne-TP Thx, as I say above, I have 4 of these thermostats, two of which on rads with the same valves and adapters, I can move this faulty thermostat to the other room with the same valves \ adapter and I get the same behavior, this is 100% isolated to this thermostat, the others all function correctly if I swap them onto this radiator. 

Have had to send pictures to TP-Link support, now I have to wait until tomorrow :-(

Cheers

Mike

  @Wayne-TP TP-Link support have just confirmed that they believe the device to be faulty and to return it to Amazon for an exchange. Nothing they can do to repair!