HI
poss problem could be the heater canister is scaled up with lime scale thus causing lack of water inside the heater canister and over heating of the smaller amount of water, are you in a really hard water area and how long has the shower been installed if hard water area and over 2 yrs installed then the unit requires descalling but this is not an easy job ie heater canister has to come out and liquid descaler used to descale inside of canister,or a replacement shower fitted. hope this helps
cliff davies, October 2007