DIAGNOSIS: The plastic speed selector ring is either broken internally or jammed with dirt/debris. It's a detent mechanism that can get clogged.
FIX: First, try cleaning it aggressively. Pull the rubber boot forward, and spray electrical contact cleaner or 99% isopropyl alcohol into the gap around the selector ring while rotating it back and forth. Work it dozens of times to flush out any grime. Let it dry completely.
IF THAT DIDN'T WORK: The internal detent spring or ball bearing might be broken or dislodged. This requires taking off the front housing, which is a more involved repair to access the selector switch assembly.
HEADS UP: Don't use WD-40 or lubricants here initially—they attract more dirt. Use a cleaner first. Forcing the ring to turn if it's physically broken can snap the plastic tabs.