I have CB24 Boiler and my water keeps running cold . ?

Over the last 20months (I am in a new built flat) I have had a plummer out 4 times and everytime they say the air pressure just needs topping up. I did this last time and it didn't work, the plummer came out and it worked, two weeks later its running cold again. I've opened both valves the water pressure is on approx 2, but still I have no Hotwater, any ideas?