Diagnosis
The locking mechanism is failing to engage. This is usually due to food residue or a worn plastic latch on the blending arm.
How to Fix It
Unplug the unit. Press the release button and pull the arm off. Clean the square drive socket on the motor unit and the corresponding square drive post on the blending arm with a dry brush. Ensure no debris is blocking the latch inside the arm's collar. Reattach, listening for a solid click.
If That Didn't Work
The plastic locking latch inside the collar of the blending arm is likely broken. This part is not user-serviceable. You need a replacement blending arm.
Heads Up
Forcing the arm on can strip the drive. Always press the release button before attaching or detaching. The click should be distinct, not a grinding sound.