Possibilities:
1. There may be a thermostatic mixing valve (TMV) fitted in the supply to the shower. These are a requirement for new builds, and may be fitted anyway. The hot water can never exceed the temperature at which the TMV is set. Resolution would be to reset the TMV, but requires thermometer.
2. The shower may have been set to a maximum temperature which is lower than you like. If pressing in the button on the side of the knob (your photo. 1) doesn't enable you to increase the temperature to one you like, you may have to adjust the maximum. The adjustment is shower specific, and unless you have or can get the installation instructions for the shower it is likely to be trial and error.
3. As SimonG has suggested, the thermostatic valve cartridge in the shower may be faulty. Take the shower apart (after isolating water) and clean in white vinegar of Kilrock. Reassemble with silicone grease and see if it is better. Also requires manual.
4. If you have a combi boiler, it may not be supplying hot water at a temperature sufficiently high to give you the shower you want. Hot water at the boilers maximum flow rate will always be cooler in winter, as the incoming cold is colder than in summer, and the boiler can only raise its temperature by (normally) 35 degrees. If you turn down the flow, and the temperature increases, suggests boiler.