my radiator is the weakest amongst all radiators in the flat

Option 1. Make sure when bleeding the radiator to let some water come out and see how dirty the water is if it keeps coming dirty you might have some blockage in there and might have to take the readiator off turn it upside down and flush with a hose if available. Option 2. There is a little valve usually on the left side or opposite of the thermostat try and twist it quarter turn at a time counter clockwise and wait about 30 minutes between and see if there is any improvement, sometimes the valve has a cap on it try to take it off and use some pliers to twist it. If neither option works most likely you will need a hand from a Plummer. Thanks Mantas

System could be air lock or pressure on boiler has gone down system may need balancing properly

I would check the heating system to make sure if it's a combi boiler the pressure isn't low first. Then isolate the radiator each side & flush the radiator out, refix then try again. Another way is the turn the other radiators off & leave the one that isn't working properly on this should hopefully sort the problem. If that doesn't work could mean a new radiator.

Plumber, heating system needs balancing