Electric showers control the water temp by increasing or decreasing the flow of water over the element. All electric showers can be turned to a point where the water is almost a dribble but you could not use it as a shower because apart from the flow, the water would be too hot.
If you have a good flow of water when the knob is turned to cold, but cannot get a good temp without reducing the flow to just a dribble suggests one of the heating elements is not working.
If you have a half power and full power control button, see how the different settings alter the temp.
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