Few comments here ...

You got someone on this thread telling you to disconnect a feed to a rad to see if it has flow .. ravin mate .. If you do that with 1.5 to 2.0 bar pressure behind it .. you maybe know what a mess you might make That's 20-28psi .

Remember that a pump ... is really a circulation device just like a car water "pump" and designed to maintain a certain head of pressure according to the spec. You can stop a pump with your fingers .. that's normal with an inductance motor. They usually leak from the gland .. or seize up when not in use for a while. If it has inhibitor in the system and the pump is running without noise, starts ok, and also heats all those other rads ok .. except those odd two .. and has only been in use for 3 years, its unlikely to be the cause.

Looks like you already have inhibitor in the system .. you need to make sure you dont lose it all.

If you need to dislodge a lot of air, take the bleed plug right out .. but have it ready to replace and a cloth on the floor ready! lol

I had a problem similar to this on a fully-pumped system, after a builder had added an extra rad in a friends upstairs bedroom a few years back. He'd managed to get a big airlock under the floorboards .. mainly due to system design. I had to back-fill it with a hose from the drain point OK after a struggle.

Is this system 15mm copper .. 10mm Microbore .. or 8mm Microbore at the rads?

Oh! One other thing .. You check the flow with a temp check .. not flow of water. It's a pressurised system, so you will get a flow from a broken connection on supply AND return sides!