Shower running cold?

I have a Triton aspirations team shower. When the hot button on the shower is pressed water rows from the shower head as usual, for 10 secs. The hot Lithgow goes out and the cold button lights up old water still flows a normal pressure, the red power on light flashes. I have checked the filter, clean as a whistle. The water pressure to our business hospital not changed, and water dribbles out in the region of the filter assembly. Any ideas Brian Lynch