The heater works on an outdoor sensor.

The heating at your complex is set to the outside temperature.
This means the colder it is outside, the warmer the heating becomes. In spring and autumn when the days are relatively warm and the nights a bit colder, the system does not get very warm (yet). So this does not mean it is faulty.

If it gets colder during the day, your heating will get warmer.
Is this not the case? Then submit a repair request. We will then see if there is another cause. 
Is the heater getting warm locally (knob side at the top)? Chances are that we then need to bleed the radiator. Please also contact us for this.