not getting hot water in my electric shower?

is a Gainsborough electric shower. Delux 10.5 Kw
i'm not getting hot water in my electric shower.
i have replace my old one assuming there was something wrong with it.
bought a new one.
but still not hot water.
there is electricity coming to the shower, so i assumed there's no an electric problem.
there's is enough water pressure. in cold water the pressure is fine an no problem with that.
but when turning the dial to the hot secction, the pressure drops a lot and the water comes cold.
anyone can help me. ???
please thanks