Floor heating not working, any ideas?

Hi all,

I recently moved in this apartment, built in 2013, which has floor heating.

When I first visited the apartment (a month before moving in) the heating seemed to work fine (it was actually so warm I was sweating inside).

After I moved in, I noticed the heating is not working well at all. Specifically:

1. The main living area (~45sqm) is controlled by a master thermostat, and despite the temp I set, I get max 21c,
2. Same living area seems to have uneven warm areas on the floor,
3. Two bedrooms are controlled by a different thermostat system, which only open the valves on the return pipes (so they seem tied to the overall temp set by the master thermostat in the living area)
4. Master bedroom floor is cold, despite the thermostat being cranked to the max,
5. Smaller bedroom floor is lukewarm, despite the thermostat being cranked to the max.

Called the landlord's contractor for looking into it, but all they said was that the system is supposed to be working, and that it just takes time for it to warm up.

After 3 days of thermostats set to max, nothing has really changed.

Here is the setup:

From left to right, the return pipes are:

1. Cold-ish (Living room 1)
2. Lukewarm (Living room 2)
3. Cold (???)
4. Cold (Bedroom 2 — I suspect Master Bedroom)
5. Cold-ish (Bedroom 1 — I suspect Second Bedroom)
6. Lukewarm (Bathroom)

The pipes here seem to be numbered as such:

1. Supply heat
2. Supply cooling
3. Return heat
4. Return cooling

I have no idea what's the possible issue - any guess?

I mentioned the possibility of some air in the pipes to the contractor, but he said it's not possible since it's a closed loop.

Also worth mentioning: previous tenant told me they never really turned on the heating in the bedrooms in 6 years, so I can't tell if it has ever worked or not.

Any lead is really appreciated, yours truly here is not faring well with the cold and humid Netherland winter.

Cheers!