Diagnosis
This is a design flaw with this model. The hollow arm has a small opening where the shaft enters, allowing pureed food to be forced up inside.
How to Fix It
Immediately after use, before food dries, detach the arm. Rinse the outside. Then, hold it under a strong stream of hot water from your faucet, aiming directly into the square drive socket at the top. Let water run through the arm and out the bottom around the blade shaft to flush debris.
If That Didn't Work
Use a long, narrow bottle brush designed for baby bottles. Gently work it down through the top socket. Do not force it.
Heads Up
Never store the arm attached to the motor unit if it's not completely dry inside. Trapped moisture can migrate into the motor housing and cause electrical failure.