DIAGNOSIS Leakage from the bottom of the cup indicates a failed primary seal. The silicone gasket under the blade assembly is either damaged, incorrectly installed, or has food debris preventing a tight seal.
FIX Disassemble the cup. Remove the blade assembly. Inspect the black silicone gasket for tears, nicks, or permanent deformation. Clean the gasket groove in the cup and the gasket itself. Reinstall the gasket, ensuring it is seated evenly in its groove without twisting. Reattach the blade assembly and tighten firmly by hand.
IF THAT DIDN'T WORK The plastic threading on the cup or the blade assembly may be cross-threaded or cracked. Hold the cup up to a light and inspect the threads for hairline cracks. A cracked cup cannot be repaired.
HEADS UP Overtightening the blade assembly can warp the gasket and cause leaks. Hand-tighten only. Do not use tools. Always assemble the cup with the gasket dry, not lubricated.