I can only get heating to the radiators upstairs, not downstairs

sorry but we know nothing about Dunkirk boilers contact a local plumbing heating company

Upstairs is usually for winter heating where downstairs is usually for summer cooling.

close ceiling vents
the hot air down stairs rises through the vents and adds to the upstairs heating
same as hot air goes up the stairs to the upper rooms

yes pump is not working it could be stuck try tapping it may start moving

check out the pump see if it is working , or has air in it , sounds like the pump to me

Take an Amp draw on all heaters and compare. If within 5%, they are working fine but you may have more open area downstairs than up.

Hi, sounds like you determined the problem is not with the thermostat. That leaves the furnace that serves downstairs. What type heating system do you have?

This is a flow problem within the system and you have ruled out the two major problems already.
asuming the pump has been fitted in the correct way round and the pump isolation valves are open fully. have you tried closing down the radiator valves upstairs and forcing the flow around the downstairs circuit? also is there any zone valves on the circuit not opening.
Is it a conventional type system with a header tank ?

Some of this depends on where the air handler/furnace is located. Most often in two story houses, it's in the attic upstairs, or in a closet upstairs. A duct routes air from the furnace to the downstairs supply duct, so it is a distance from the air handler.
When the unit first initiates a heating cycle, that duct is going to be full of cool air, and the duct itself will be cool. So the furnace has to run long enough to push the cool air out, as well as warm the duct up, before your going to feel warm air at the registers downstairs.

Keep in mind, heat rises. And, your thermostat only monitors the immediate area where it's located. In your case, upstairs.
Two story houses are problematic because of this. And one way around it is to have a damper system installed that distributes the air upstairs or downstairs based on a thermostat located in those spaces. There would be 2 dampers, and 2 thermostats (one upstairs, one downstairs). Each stat would control a damper, and the call for heat or cooling.

Let me know if you would like to consider a system like this, and I can point you to components to use. I've put several system like this in.

Hi Leehardcast,

Yes the problems are related. The system is low on water. So now instead of water you have air in radiators which is easily compressed when it heats up. Then pressure loss when it cools down.

Add some water to your system until you have 15lb pressure and bleed out upstairs radiators at the same time.

Please rate this response and Good luck, Phil