Only Hot water?

its difficult to say without knowing what brand model and type . But if its a Thermostatic mixer and you have demonstrated to yourself that you have the required pressures at the valve inlets , either you have reversed the hot and cold outlets . Hot is normally on the left. Erratic behaviour from thermostatic valves is usually pressure related or crossed supplies. Also satisfy yourself that the service filters are clean. good luck..

Alan
December 2008

Quite often the mixer input supply pipe gets an accumalation of dirt and this stops / slows the flow
If you can get at the input side of the unit you will find each ( hot and cold ) input has an in-line filter to stop gunge getting further to the mixing unit --- Please be advised this necessitates interrupting the supply / accessing and cleaning - and as tiling is probably present it is NOT always an easy fix

pc6493
December 2008

Can you trace the pipe back from the cold tap? If so, check if there is an isolator in the pipe that has been turned off (or nearly off) - if there is one turn it on! If no isolator follow pipe back and try and find another cold tap on the same pipe run and see if that works. It is always possible that the tap on the shower control has seized or the washer stuck - especially if your water pressure is low. If that is the case - or you've run out of other ideas - turn your water supply off and dismantle the tap that isn't working - you may find the problem within!

Richard
December 2008