Mixer Shower Not Hot Enough

[plumbandplumber]

I have a Gainsborough thermostatic shower mixer valve. The water coming out of it isn't as hot as it was, not really hot enough for a shower. Strangely, the hot pipe in the cupboard behind valve that feeds water to shower valve is very hot. When I run the bath cold tap (the bath has it's own mixer tap) quite fast, the shower runs hotter.

Would be grateful if anyone could tell me what's gone wrong. Many thanks.