The heater matrix bleed nipple gets grotted up, or more accurately oxide & other detritus builds up around the nipple end. Set your stall out prior to clearing this out, you’ll obviously want to get the nipple back in once clear, you won’t want coolant pissing out all over the place whilst to find the bloody thing. Tuck it in a fold of the towel you’ll need to dry up after.
Remove the nipple & see above & with an old bike wheel spoke or similar poke the grot out. Last time I did this for a friend there was a wonderful fart, followed by a steady flow of coolant. His problem was cured, I hope yours will be also.
M