I know this is a joke, but usually it's caused by old cartridges. If you replace the valve cartridge then it will probably mix better. This is usually from old/bad seals. This is why new valves mix fine, but as the seals age, mixing performance gets worse.

Pulling old cartridges can be a pain in the butt so probably best to call a plumber if you don't like diy adventures.

https://www.home-barista.com/ Is pretty good for coffee related topics, specifically espresso. It's a lot more technical than /r/espresso