Upstairs Radiators not heating up...

a well fitting screwdriver, and a wrench for the valve body, more PB, Tap(aggresively gently, tap, tap, tap) on the screwdriver handle to shock the vent screw,

or try loosening the entire valve body, and look for venting air, or water, at the threads, loosen, don't remove completely, loosen slightly,

but first, at the boiler, what pressure do you see on the gage? post that picture,

Try adding some water and see if the pressure gauge moves. Should be about 12 psi cold. (Black numbers on the gauge)

Pressure regulator, however it only works if the cold water supply is applied to it. The feed water valve may be closed.

For one thing, you may simply be low on pressure, which may have led to problems with air in the upstairs radiators. This is solvable…

I note, however, that you have a compression tank — that lovely big blue grey tank hanging from the overhead. They are an excellent way to control pressure — don't let someone convince you that because it is old fashioned you need to change it — but they can get waterlogged. However, if when you the system heats up you don't see that much change in pressure — a few pounds is fine — it's OK and we can worry about it later, if at all. As @delcrossv suggested, try adding a little water and see if the gauge moves.

Now on controls. I'm intrigues. I wonder if, in fact, you actually have only two zones since I only see two pumps — and if the upstairs one may be controlled by only one of the two thermostats.

Get yourself one of these 0-30 psi water pressure gauges and some brass threaded adapters at your local Home Depot plumbing section to go from the 1/4" NPT thread on the gauge to the 3/4" Garden Hose Thread (GHT) drain valve on your expansion tank. Connect the gauge to the expansion tank drain valve and open the valve to get an accurate reading of water pressure in the system. Then add water to the system until you get 12 psi.

https://www.amazon.com/gp/product/B07T74PNQP/ref%3Dppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

This is what I use to check our system pressure, because our boiler gauges are inaccurate.

You can get water pressure gauges that already have the garden hose thread fitting on them, but they tend to be 0-200 psi gauges which don't have good resolution at the low end of the scale, so don't get one of those.

That is correct. The 0-100 psi gauge you showed should work, even though the resolution on the low end isn't great. Thread it onto that valve hand tight, like a garden hose, and open the valve handle just a bit to get a pressure reading. You can leave the gauge on while you adjust the system pressure to 12 psi. Then close the valve and remove the gauge and save it for future pressure checks.

Yes, that's perfect.

If you have the handy hose gauge, you don't have to worry about replacing the one on the boiler.

I see you have an Airtrol boiler fitting, the other end of that pipe should be going to the corresponding Airtrol tank fitting on your compression tank.

Looks like this:

Good job. As @delcrossv said, your expansion tank will work best with an Airtrol tank fitting (ATF) that is designed to prevent air-laden water from escaping the expansion tank and getting back into the system, where it can then bubble out into radiators.

You can get them new at supplyhouse.com, or get a new old stock one on eBay for half the cost. Beware that they come in different lengths, as indicated by the dash number. For example, an ATF-12 is sized for a 12-14 inch tank diameter. So measure your tank diameter and get an ATF with the corresponding dash number.

The ATF needs 3/4" diameter pipe to work properly, and it also needs to be pitched up at an angle. So the 1/2" straight pipe you now have to the expansion tank would need replacing.

Once installed correctly, you should never need to bleed your radiators again, as long as you maintain proper pressure.