The fuel nozzle in your hot water pressure washer is a wearing item. It needs to be changed regularly. When you change it depends entirely on the quality of fuel passing through it. As the nozzle wears the droplets which are atomised become bigger until they will no longer ignite easily. The symptom of this is flames appearing up the chimney. As soon as the flames are visible half way up the square section, change your nozzle and your filter, as well as removing the fuel tank and draining out the sediment and water that will have accumulated.
Another reason for the water not getting as hot it used to, is a damaged cone. If you strip a fuel nozzle you will find that there are three very small grooves that the fuel travels along before it leaves the nozzle.
Dirt can block one of these grooves which forces most of the fuel out of one side causing a lopsided flame, which will in time oxidise the metal causing it to flake and drop into the bottom of the coil. The flame will burn mostly on one side of the boiler and even heating of the coil will not occur leading to a loss of heat output. Other symptoms include a hot outer casing and a partially melted fuel tank.