DIAGNOSIS The sealing gasket (a black O-ring) on the bottom of the filter reservoir is misaligned, damaged, or has debris on it.
FIX Empty the pitcher. Turn the reservoir upside down. Locate the circular black gasket seated in a groove around the filter stem. Remove it, rinse it and the groove under cold water. Re-seat the gasket evenly in its groove. Ensure the filter is tightened hand-tight.
IF THAT DIDN'T WORK Inspect the reservoir itself for a hairline crack, especially around the filter housing or spout. Cracks are not repairable.
HEADS UP Do not overtighten the filter. It can distort the gasket or crack the plastic housing. Hand-tight is sufficient.