DIAGNOSIS Dirt, sawdust, or dried grease has jammed the switch mechanism inside the housing.
FIX Use compressed air to blow around the base of the switch. Apply a small drop of light silicone spray (not WD-40) into the switch seam and work it back and forth.
IF THAT DIDN'T WORK The switch detent spring or plate may be damaged. This requires disassembly to replace the switch assembly.
HEADS UP Forcing the switch can break the plastic selector lever. Apply spray sparingly to avoid attracting more debris.