Why won't my radiator capture hot.? just one radiator within the whole house does not get hold of...

just one radiator within the whole house does not get hold of hot. I have tried bleeding it and it starts getting hot after when I stop it goes cold ageain. Should I bleed radiators near the heating on or past its sell-by date ? What am I doing wrong ?
What sealant do I use for installation of...
Answers:    radiators need bleeding regularly..as nouns gets trapped at the top and stops the flow..trial radiators sometimes get blocked near residue from the sealant..old radiators achieve blocked by rust..which i think I don`t know the case beside yours ..also there is a regulator at the bottom which adjust the temperature..so first bleed the radiator..second isolate the radiator concerned and drain the hose down from the bottom ..this will clear any obstruction..variety sure the control for temperature is unstop full..once this is done close the runoff tap..enlarge the bleed valve..and re-fill by un-isolating the radiator..once the dampen comes out of the bleed valve close the spout and it should work perfectly..
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Bleed ALL the radiators.