Diagnosis
The depth adjustment locking lever is not fully engaging the rack gear. This is usually due to sawdust buildup or a worn lever.
How to Fix It
Unplug the battery. Rotate the depth adjustment ring to its highest position. Use compressed air to blow out the rack gear teeth and the area around the black locking lever pivot. Cycle the lever several times. Re-tighten.
If That Didn't Work
The plastic locking lever itself may be cracked or worn. Visually inspect it for cracks, especially near the pivot pin. A broken lever requires replacing the base assembly.
Heads Up
Never try to use the router if the depth lock isn't secure. A slipping bit will ruin your workpiece and is dangerous. Always verify the lock before starting a cut.