Two showers upstairs at my house. One in the en-suite the other in the main bathroom. The one in the main bathroom is a bog standard mixer straight from the bath taps. Flow fine on both hot and cold. The en-suite shower has the hot and cold pipes going into a mixer valve. THe flow coming out is barely a trickole ..more a drip, hot or cold or combined. Checked to make sure there are no inline valves that are closed. Is the thing likely to be gunged up or is there a way of adjusting the flow through these things?
John