First hold the hot inlet and see if the water temperature is hot enough, the shower temperature will always be a few degrees cooler then the temperature at the inlet.
If this is OK you could remove the temperature stop ring (part4) http://www.showerdoc.com/shower-spares/triton/TRITO-PARENT-137-Triton-Exe---Pressure-Compensating-Bar-Mixer and see if you can tirn the temperature up a little further.
If the temperature will not go up then you will have to replace the cartridge.
It may be the inside plastic of the knob that is broken or the on off valve that is faulty.
Knob pack.
http://www.showerdoc.com/TRITO-83307390-Triton-Closure-Knob---Exe-83307390
Flow valve
http://www.showerdoc.com/TRITO-83307320-Triton-Mechanic-Headwork-(flow-Cartridge)-83307320