Why do i need to bleed raidators all the time!?

[philv]

Hi,
I seem to need to bleed radiators quite often (usually the ones upstairs).
After a couple fo days ro so (i think) i again have trapped air.
I turn the central heating off before bleeding.
I bleed the ones upstairs last.
Anyone have any ideas?
This started before the cold snap set in recetly.
Thanks,
Phil.