Smart thermostat not reading temperature correctly

I have used a Netatmo thermostat in my main home for over 10 years.  A few weeks ago I replaced the NEST Gen 2 thermostat in my shared holiday home with a new Netatmo thermostat using the option to hard-wire the thermostat to the heating system. 

The device is successfully connected via the app and does turn the boiler on and off.  However recently I have seen that the measured temperature no longer rises in line with the room temperature where the Netatmo is installed.  Instead the displayed temperature is very slow to change and consistently reports a lower temperature than is actually the case. This causes the system to run the boiler continually unless I manually turn the heating off or set the scheduling so the heating is only on for short periods. 

I have used the "true temperature" function to adjust the measured temperature so it matches the room temperature but the Netatmo thermostat still reports lower temperatures than is really the case.

Does anyone have a suggestion on what is going wrong?  The device was purchased from Amazon in September and fitted last November.  Thanks.