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Let's assume that the brew pressure and temperature are correct, the equipment is clean, you have a quality grinder, and the coffee beans are fresh. A quick checklist for diagnosing an improper extraction follows.
Remember to dry the basket before dosing, and double-check the distribution is even. Verify the dispersion screen and water jet breaker are clean and clear. If the water in your area is hard and the machine hasn't been descaled regularly, check for scale build-up.
Correct the grind. If you own a Mazzer Mini, Cimbali Junior, or Macap grinder, check out the site's reviews for general hints on adjusting these premium choices.
Grinds dry out rapidly and extract much more quickly than fresh grinds. A mixture could create pockets of unevenness. Either run the grinder for a couple seconds and discard the first grinds exiting the chute since the last shot (when dosing by volume), or always brush the chute clear after grinding (when dosing by weight). Also remember to grind for couple seconds after changing the setting to clear the chute, and then empty the dosing chamber.
Not enough coffee, uneven distribution, or canted tamp. Try overdosing (dose the portafilter until it is about three-quarter full, tap it gently on the grinder fork two times to settle the grinds, dose the remainder, then distribute and tamp as usual). Double-check that the tamp is level. Verify the dispersion screen is clear and water exits from it evenly.
This could be caused by too much tapping or rapping the side of the portafilter after the first tamp; consider a Staub tamping style. Another possibility is the top of the puck is grinding against the dispersion screen during the portafilter lock-in; verify there is adequate clearance to allow the puck to expand up to meet the dispersion screen. If channeling is near the handle, remember to rotate the portafilter while dosing to avoid front-loading the grounds and confirm you are not "working around" the handle when distributing.
Baristas endlessly debate the merits of flat-bottomed versus convex-bottomed tampers. Careful readers will note that David Schomer switched sides between his original article in 1998 on packing techniques and the 2004 update that advocates a domed bottom. His thinking is that a slightly domed bottom will help push grounds to the edges for a better seal without unduly thinning out the center of the puck. Since I do a Staub tamp, the convex tamper doesn't offer much for me in terms of improved edge adhesion. On the other hand, those in a rush may prefer Schomer's less tamp-happy compress-and-polish approach with a modestly convex tamper. Reg Barber makes a very nice one with a gently domed bottom and he'll even re-size the tamper base to precisely the basket's diameter for a reasonable $15 fee.
Still having problems? Then check out Banish Uneven Extractions with the WDT! Now that you've got a firm idea for what could go wrong, continue to the next section to see what the result looks like. Be forewarned that it won't be pretty.