Could still need bleeding. As it's so tall it's more likely to retain air at the top as most other rads will be lower. I've never had to bleed one but assume it's not difficult and bet info is on YouTube somewhere
I would check the fluid level in the Alde tank, if not I think as Lenny said air is the problem so yourl need to bleed the air out. Bit odd though we have the same system and same radiator ( which is only a pipe under the rad like cover ) and never had to top it up or bleed it.