Diagnosis
Dirt and metal debris have packed into the selector switch mechanism.
How to Fix It
Unplug the battery. Use compressed air to blow out the switch area from multiple angles. Work the selector back and forth while blowing. Apply a single drop of light machine oil (like 3-in-1) into the switch seam. Cycle it dozens of times to distribute.
If That Didn't Work
The internal detent spring or plate may be damaged. This requires disassembly. Do not force it.
Heads Up
Never use grease. It attracts more debris. If grinding continues, stop. Forcing it can break the switch housing.