Diagnosis
The silicone sealing ring is not seated correctly in its groove on the extractor blade assembly. Or it is worn and no longer sealing.
How to Fix It
Disassemble the blade. Clean the groove thoroughly. Any food debris will keep the ring from sitting flat. Press the ring evenly into the groove around its entire circumference. It should snap in and sit flush. Reassemble the blade onto the cup.
If That Didn't Work
The sealing ring is likely stretched, torn, or hardened. It needs replacement. Order a new silicone sealing ring for the NutriBullet Pro Plus extractor blade. They are cheap and essential.
Heads Up
Remove the sealing ring every time you clean the blade. Dry it completely before reassembly. A wet ring can slip out and cause a leak.