Sour espresso
Sour espresso often means under-extraction. Grind finer, increase contact time or check that your water and machine are warm enough.
Also check beans: very light roasts can taste sharp if your setup is not ready for them.
Bitter espresso
Bitter espresso can come from over-extraction, too fine a grind or water that is too aggressive for the coffee.
Grind slightly coarser, reduce yield or try a bean with a softer roast profile.
Channeling
If espresso sprays, blondes unevenly or tastes hollow, the puck may have weak spots. Distribute the coffee evenly before tamping.
A better grinder, WDT tool and consistent tamp can all help, but start with calmer puck prep.
When to stop adjusting
If the coffee tastes pleasant, stop chasing perfection. Espresso numbers are helpful, but the cup is the goal.
Build repeatable habits before buying more tools.