This can be caused by a couple of things.
If the boiler has gone over the 3 bar threshold and opened, then muck can be caught under the seat of the valve and is not letting it seal properly.
It could be that the expansion vessel that takes up the extra pressure when the water heats and expands, has lost its pressure and needs represureising.
The expansion vessel has become porous and the primary heating water has seeped through to the air side of the vessel not allowing the primary, (heating) water to expand and blowing the pressure release valve open.
You need a gas safe engineer to investigate it.
Good Luck
Mike

Your auto fill is stuck open, is there any way to shut off the supply of water to the boiler?