Option: That is most commonly caused by badly setup derailleur limit screws and these will require modifying. It can even be due to a bent derailleur hanger Pedal the crank ahead and manually drive the derailleur to the largest cog. If this motion will cause the chain to come back https://indexingbikegears93580.howeweb.com/33438734/the-fact-about-indexing-bike-gears-that-no-one-is-suggesting