This is electric underfloor heating you're talking about, right?
If so the thermostat 'clock' should have a couple of settings, one being to work on the air sensor inside the thermostat controller, and the other to work off the sensor under the floor. The default is to work off the floor sensor first and you can switch it to work off the floor sensor.
What's the make of it?
Did you see for sure it was installed?
Did your plumber do the tiling and install the ufh too?