Hi,
I have a shower mixer which operates left hand side water flow and right hand side temperature. No problem for shower but for bath it isn't hot enough. The water coming from the boiler to the mixer is hot enough but not when it comes out of the tap, even on max temperature.
Eeza.... since the shower is Ok, I guess the thermostatic cartridge is working properly.
I can only think that the diverter needs to be dismantled and cleaned out. It may simply be sticky.
I've a Combi Boiler but it's not that. The water is hot enough upto going into the tap mixer. Then it seems to be met with cold water which reduces the heat. Just feel the heat on the pipe that feeds into the tap.