Diagnosis
Consistent bitterness points to the brew water being too hot, overheating the coffee grounds. The PID-controlled thermoblock may be over-heating or the group head may need a cooling flush.
How to Fix It
Lower the brew temperature via the menu settings. Start by reducing it by 2°C (4°F). Before pulling your first shot of the day, run a short flush of water through the group head without the portafilter to cool the shower screen.
If That Didn't Work
The machine may be set to a pre-infusion time that's too short, causing channeling and uneven extraction. Increase the pre-infusion duration in the settings menu to allow more even saturation.
Heads Up
If you've recently descaled, residual descaling solution in the lines can cause a chemical bitterness. Always run several full tanks of clean water through both the brew and steam systems after descaling.