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.
you cant stop new pumps with a screwdriver - it takes a hell of a lot of pressure on that pump to do that
its not uncommon for a combi pump to fail after a few years - some last for the life of the boiler but others can fail after a year.
if you really think its an air lock drain the whole system down, take every air bleed from the radiators so all the air os out of the system and start again.
fill the system to 1.5 bar and bleed all the radiators starting from the bottom all the way to the top. if after all this you still have problem heating the property CHANGE THE GOD DAMN PUMP
i work on combis every day and see on average 4-5 a day. this sounds very much like a pump problem and not an air lock. i have never had an air lock on a combi system....ever