On 07/05/2019 16:06, John Rumm wrote:
Remember that if its an upstairs rad, then the pressure at the RAD could
easily be half a bar (i.e. 15' of head) lower than that seen at the
boiler. Many boilers will work down to 0.5 bar, which could equate to no
nett pressure at all on the first floor.

That could be it, the rad in question was on 1st floor.