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