Bathroom radiator not getting hot when hot water is on

macgyver

I have a old style potterton condensation boiler heating system The radiators in my two bathrooms/toilets used to get hot when ever the hot water was turned on. Now the one in downstairs Cloak-room gets hot with the hot water but the upstairs bathroom radiator does not. I noticed it as I recently turned off the central heating. As the radiator gets hot when the central heating is on but not when only the hot water is on. All other radiators are working normally (turn hot only with the central heating).
This was not the case earlier.

Any suggestions or things I should look into?

Thanks