Diagnosis
Fine abrasive dust (drywall, concrete, MDF) has gotten inside the switch mechanism and is acting like grinding paste.
How to Fix It
Move the switch back and forth firmly several times. Then, use compressed air to blow directly into the gap around the switch slider. Follow up with a small shot of dry lubricant like PTFE spray (Teflon) into the same gap. Work the switch to distribute it.
If That Didn't Work
The dust has damaged the plastic detent ball and spring inside the switch. The directional switch is part of the main trigger housing assembly. It is not a separate field-replaceable part.
Heads Up
Forcing a stiff switch can break the internal plastic lever that actuates the motor polarity. If it breaks, the drill will only work in one direction.