shower runs eratic

[red man walking]

this may seem vague but a customer says his thermostatic shower (mains cold, unvented hot fed) runs too hot when another tap in house is opened. The company who fitted it came back (cos it did it from new) and upsized some pipework. He says it was slightly better but not right. Shouldn't the shower compensate itself with it being thermostatic or am i missing something. I have not been yet to look at it but would like to take some sort of knowledge with me!!
Any thoughts would be appreciated

 

[uug197h]

the flow ristrictor could be missing (small plastic disc in the shower hose( the hose to the shower head))