DIAGNOSIS: Grime (sawdust, drywall dust, adhesive) has gotten into the sliding switch mechanism and turned into a paste, or the detent spring is binding.
FIX: Spray a liberal amount of electrical contact cleaner or isopropyl alcohol into the seams of the switch while sliding it back and forth. Do this with the battery removed. Work it dozens of times to dissolve and flush the gunk out. Let it dry thoroughly.
IF THAT DIDN'T WORK: The switch might be physically damaged internally. The switch is a separate module that can be replaced, but it requires soldering skills to remove the old one and install the new switch board.
HEADS UP: Don't lubricate it (with oil or grease) until you've cleaned it first. Lubricant will just trap more abrasive debris and make the problem worse long-term.