Radiator , every one is fine except one !!!

Basically , combi boiler , 8 radiators in total in the house , all other radiators are fine , except one , it works fine , but over say a month it tends to end up full of air ? so i use the key to open the valve and let the air out , usually quite a lot and then its fine for say 4 weeks ?!!? the rest are perfect , this radiator in question is on the same floor as the combi boiler and is perhaps only 3 meters away , but why only 1 radiator out of all of them have this problem ?
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