The motor is wearing out or the plastic gear(s) is broken. Experience has taught me...it's usually the plastic tooth gear....it could be the actual motor or the rail/track gear.
Another possibility...the switch is stuck....I use air and blow out the crevices in the switch...the "can air" used to clean out computer keyboards...if I use the shop air...I make sure I am at least a foot or so away...too high pressure....you'll blow or break the switch apart.