hot, warm and cold tubes on vertical rad

Hi,

I've got an upstairs radiator with 12 vertical tubes in a kid's room. When the CH has been running for a while all other radiators are hot (inc another one in the same room), but on this one - only a few of the tubes are hot, some tubes are warm and some are cool. There is no particular order to the tubes that are getting hot. Flow is hot, return is warm.
It is a fairly new radiator.
I've bled it and gently banged it.
I can hear quiet dripping sounds inside!? Does that mean air still or an airlock (if there is such a thing)?

Anyone had any similar problems. All comments gratefully recieved.
Thanks

is the radiator completly level? - if not there may be air at the top prventing the water to flow properly around tubes (depending on configuration of the rads water ways).

just a guess

Thanks for reply bster.

I did wonder about that, but it is level (within a mm or two!)