Your shot comes out in five seconds, thin and sour, with a weedy pale crema. This is channeling: water finding a path around the puck instead of through it.
This troubleshooting guide walks the exact causes and fixes, from grind to distribution to tamp.
The classic causes
Grind too coarse - the number one cause. Fix: grind finer.
Distribution: a crater or high side in the puck gives water a bypass. Fix: level and distribute before tamping.
Tamp: a tilted or weak tamp lets water channel. Fix: level, firm tamp.
Fast diagnostic flowchart
1. Is the grind too coarse? Then grind finer.
2. Is your dose right for the basket? Underdosing channels.
3. Distribution: use a WDT tool or distribution finger.
4. Tamp level and firm.
If none work, clean the group and reset your grind step.
When it is the machine, not you
If your machine reads 9 bar and extraction still gushes at a reasonable grind, check for a blocked shower screen or a worn group gasket - both are cheap, DIY repairs that fix sudden behavior changes.