Why is my espresso shot running too slow?

Shots running too slow (over 40 seconds, or barely dripping) produce bitter, over-extracted espresso. Causes: grind too fine, dose too heavy, channeling, or coffee puck too dense. Fix: grind coarser. If you're already at a coarse setting and still getting slow shots, check that you're not overdosing for your basket size. A cracked or deteriorated shower screen can also cause uneven flow — inspect and replace if it looks damaged.