Thermostatic shower not getting hot enough?

[PickANameAnyName1]

I got a new bathroom with new thermostatic bath tap/shower a few months ago. But it doesn't get hot enough. I got the instructions and took the end off and tried to adjust the set temperature bit. I think I've turned it to the hottest setting but the water still doesn't get hot. I've tried increasing and decreasing the water temperature on the boiler. That doesn't help either. I've tried decreasing the amount of water coming in to the boiler by turning down the main stop cock a bit.. that didn't work either.
Anyone know how to fix it?
Thanks

You need PigletJohn here.

Does the water get properly hot in all of the other taps in the house?! If yes, then I expect there's a problem with the thermostat in the shower, I wouldn't be tinkering with the boiler. We had a shower that didn't get hot, it was a broken spring in the setting, but it was years old.

Does it have a button to press that allows the knob to turn past 38 deg?

Yeah it does. And it just stays turned up to the hottest. I tried taking the end of the top off and turned the spring thing round so it should be even hotter... it isn't.
Other taps in the house get hot..

I'm sure you have but have you checked you are turning the thing inside the right way? I turned mine the wrong way and made it colder when I tried to adjust the temp on my new shower last year!

Yeah.. I've tried it from off to... fully.. around haha if that makes any sense haha

show us a pic of the mixer, and tell us the make and model.

Have you got a combi boiler?

Is it the bath, or the shower, or both, that are not hot enough?

How big is the shower head?

Hi,
It's this tap; www.victorianplumbing.co.uk/premier-wall-mounted-square-thermostatic-bath-shower-mixer-valve-bottom-outlet-chrome-vbs005

So it's shower and bath in one. So can't have a hot shower or a hot bath 🙁

Yes to a combi boiler.

Shower has two heads. Smaller hand held one (which is usually the one used) and a bigger, fixed, over head on (waterfall?).

I see it's quite modern, so it will have anti-scald protection. It's possible this is not working correctly. The ones I use have an override button enabling you to go past the preset temp, and also and adjustment to change the preset, which I gather you have tried. If you've followed the instructions without success, see if there is a customer support helpline.

You mention a waterfall head. It's possible this tries to take more water than the combi has power to heat. Measure the litres per minute by running it into a bucket and timing it to full. Modern combis commonly have about 30kW power (which is a lot) and can heat around 11 litres per minute at a 40degress C rise in temperature. This does mean that in winter, if the incoming watermain is at 5C, the tapwater will be at 45C, whereas in summer, in the incoming water is at 10C, the tapwater will be 50C. The faster the flow of water through the tap, the cooler it will be, because the heat has to be shared into more water.

If you have a thermometer, measure the cold water temperature at the cold tap, a hot basin tap and the hot wash tap; and the number of litres per minute flow at the bath and basin. Then turn the bath tap down to a flow of 5 litres per minute and test its temperature again. This will give clues as to whether it is the power of the boiler, or the mixer in the tap, that is causing the trouble.

We had this in our old bathroom, the temp dropped considerably in the winter due to the incoming water temp being much lower. Appreciate yours is quite new, but we ended up changing it over to a diff make (Brisbane or Triton I think) and it was much better. We tended to up the boiler water temp in the winter too. Does it get cooler, i.e. The temp setting isn't stuck?

It's possible the shower is plumbed in wrongly and the cold and hot are backwards. Although generally that would result in a freezing cold shower not just warmish.
My money's on a faulty mixer. Staying under a shower over 40c isn't going to be comfortable for long for most people. We just use the hot tap only fully open for showers with the boiler on minimum (24kw Combi) and it seems to be about right.