The new bathroom radiator simply does not get warm

Hello everyone,
I have an air/heat pump from Stiebel Eltron from 2013.
Recently, I had to replace a bathroom radiator in the attic due to corrosion at the connection fittings.
The new radiator (towel warmer) was connected, filled with water, and vented. New water at 2 bar - as before - was supplied to the system.
Since the radiator did not warm up, I loosened the valve head and checked the pin for mobility. Everything seems normal, but I still sprayed it briefly with Ballistol.
After that also brought no improvement, I first increased the flow rate at the heating circuit distributor (the radiator, like another one on the ground floor, is connected to the underfloor heating), unfortunately without success. (I now know this setup is suboptimal, but it has worked well so far).
In the meantime, I have also replaced both the supply and return valves - without success.
The other radiator on the ground floor as well as the underfloor heating in the rest of the house run without problems.
What else can I do? I am pretty much at a loss right now.

What I find missing in your list is that you also fully opened the flow valve in the radiator.

Or did you mean that with "supply and return valve"?

What I find missing in your list is that you also fully opened the flow valve on the radiator. Or did you mean that with "supply and return valve"?

Morning Jesse, Supply and return valves I meant on the heating circuit distributor. On the radiator, I have opened the thermostat valve! I don't have any other valves.

I thought so.

So, when you take off the thermostat, the pin sticks out - with it, your thermostat basically controls the reaction to the temperature level set on it.

But underneath that is the actual radiator valve, meaning the setting of how much volume flow gets through in general.

Just enter "Hydraulischen Abgleich selber machen" in any kind of search engine - then you’ll see the actual valve that sits UNDER the thermostat. It's probably completely closed... these days it’s often red and has numbers on it... just try turning it higher.

However: not too much - that also influences the flow in the rest of the house!

As I already wrote at the beginning: "Since the radiator did not get warm, I loosened the valve head and checked the pin for mobility. Everything seems normal, but I still sprayed a burst of Ballistol on it." And the setting: see photo The filling also worked by opening the valve...

I could kick myself: Opposite the intake valve, you can also shut off the return flow, and I had probably closed that as well. But I thank you for your efforts!