The underfloor heating on the first floor is no longer working

So at least you should have installed the Tacos. But whatever, just install a thermostat and open the blue valve, of course check its function beforehand.

No, you can't see them, I'm not talking about the non-existent taco setters. There are distributors that regulate the flow on the return, often with a square adjuster with a scale that you can see from above.

No, you don’t see it,

Somewhere on page 1 or 2 he still had pictures, but I also thought he had some since he posted the distributor.

On page 1 you can see the 3 distributors as they are, a independent heating installation company assembled them like this 19 years ago, why and how the devil knows, there is no scale on the distributor itself

small update I have now vented the floor 6 times

Oh yes! Venting! A completely forgotten step in the age of underfloor heating. Nowadays, automatic vents are usually installed and that’s basically it. But you have a somewhat older system. Venting an underfloor heating system is a bit more complicated than with radiators, so I assume you still have air in the system and that’s why the rooms don’t get properly warm. But of course, there could also be other problem areas where it grinds.

could it also be that the pump no longer exerts enough pressure?

Yes, of course, after 19 years the pump can also give up the ghost and only deliver low performance.

or that the valve inserts are sticking see picture 1, I noticed that some move loosely when you push the nipple in and others are rock solid, I also sprayed a bit of oil on them so they move.

Another problem. The valves, especially if they are not moved from time to time (which is your case), can seize or rust. The only solution here is replacement. Besides, we don’t know what kind of water you have inside or where the air is coming from. All of this is bad for operation.

Or is it because we have different blue caps since some were broken, where can you actually get new ones, I’ve already asked if you can buy them separately but apparently they only come with a new heating circuit distributor, those blue caps.

The blue caps only serve the purpose of pressing the pin down and thus keeping the heating circuit open. The color, shape, etc. are secondary and yes, they tend to break after some time. Plastic after all.

Try googling "manual adjuster radiator" you will find something. They might look different but have the same function. Take the standard 30x1.5 then it should fit.

But as it looks in the pictures, the flow could be set/adjusted by that (although that would be quite unusual, but who knows what was built 19 years ago). So be meticulous about how many turns you make before/when unscrewing and then turn the new ones the same way. Although this is just a guess. Possibly under the brass caps below are the usual square valves for adjustment.

no scale on the distributor itself

You don’t need one. These and also the Tacosetters are of course helpful but basically not necessary.

small update, after I bled it about 10 times and refilled water, oiled the knobs of the valve inserts and got them moving, and replaced the blue caps once, it's running again, I actually thought this was part of the maintenance or why else do they do the crap if every year they only stick their stamp on the radiator to show they were there

now I just need 5 new blue replacement caps, who can find them for me?
I already asked a seller who says he only sells them together with a new corresponding heating distribution circuit