DIAGNOSIS The main sealing ring—the large, clear silicone gasket that sits between the blade assembly and the jar rim—is either missing, damaged, or not seated correctly.
FIX Unscrew the blade assembly from the jar. The clear silicone ring should be tucked into a groove on the underside of the blade unit's plastic housing. Remove it, wash it, and make sure it's not torn or stretched out. Then press it back into its groove evenly, ensuring it's not twisted. Screw the blade assembly back onto the jar very tightly until it's snug.
IF THAT DIDN'T WORK The jar threads or the blade unit's threads might be cracked or have food debris preventing a tight seal. Inspect both sets of plastic threads for cracks or chips. Clean the threads with a brush.
HEADS UP Always hand-tighten only. Over-tightening with tools can crack the plastic or warp the gasket, causing worse leaks. Also, let any carbonation from greens or yogurt subside before screwing the lid on for travel, as pressure buildup can force liquid past the seal.