My house is a single zone system heated by oil, with convectors in
every room.

Every year at the beginning of the season, I bleed each convecto, to
get the air out.

I have one convector that never seems to want to get hot, (sometimes
it does, but not as routinely as the others). I bled it, and there's
NO air in it. The water, (cold), flows out of the bleed valve with no
problem.

Any thoughts??