Shower / Hot water probs

[a9blue]

My shower is not running as hot as it should, (thermostatic), and nor are the upstairs taps although the downstairs ones are fine. The combi is on its highest heat setting.
Can anyone help?

[oldplumber]

My shower is not running as hot as it should, (thermostatic), and nor are the upstairs taps although the downstairs ones are fine. The combi is on its highest heat setting.
Can anyone help?

Run the upstairs taps more slowly to reduce flowrates and allow the combi to heat up more, might help. If it does then you can try running the shower at a lower rate, and or try adjusting the thermostatic valve (look at the manufactureres web site for instructions how to) Could be your pipe run is a bit long to upstairs or the flow rates are to high. You may need a restrictor in the combi cold feed in to keep flow rates down, check your installers manual re pressures/flow rates

[a9blue]

Flow rates are right because a regulator was fitted, and shower has been working fine till now.
The hot water tap near to the combi gets up to temperature when fully opened, but the shower and upstairs tap seem to be getting steadily worse.
Is it possible that the diverter valve is not working properly and would a chemical flush help?
Excuse my ignorance.