Diagnosis
Concrete dust, drywall dust, or metal shavings have packed into the switch mechanism.
How to Fix It
Remove the battery. Use compressed air to blow around the switch from all angles. Work the switch back and forth while blowing. Apply a small drop of electrical contact cleaner into the switch seam, not a lubricant. Cycle the switch dozens of times to work it in.
If That Didn't Work
The internal detent spring or the switch paddle itself may be broken. This requires disassembly to replace the switch assembly.
Heads Up
Forcing a stiff switch can break the plastic paddle or damage the internal contacts, killing direction control.