Unfortunately its not that simple,tests would have to be carried out to establish where the fault lies whether it is the PCB or an element. Electric showers work by producing a fixed amount of heat the temperature variation is achieved by altering the amount of water flow through the shower,so the hotter you want it the slower the water.
To get to the state you have i would say an element has gone but only testing can prove it
toptrish
April 2014