When a mechanical rear derailleur hesitates between sprockets, the instinct is often to turn an adjuster. The problem may instead lie along the cable route. The shifter pulls and releases the cable; excessive friction inside the housing prevents the derailleur from returning smoothly to the intended position. A useful clue is hesitation in both shifting directions, rather than trouble confined to one side of the cassette.
Inspect the housing ends, junctions and tight bends. Grit and moisture can enter the housing, while a rusty or frayed cable adds resistance. A crushed housing or damaged end cap can obstruct movement too. On bikes with exposed cable runs, check guides beneath the frame, where dirt may be easy to miss. A corroded cable and housing with damaged ends are better replaced than repeatedly adjusted.
Before replacing parts, distinguish cable drag from other faults. After a fall, check whether the derailleur hanger is bent. If the chain skips mainly on the most-used sprockets, assess chain and cassette wear. Accurate indexing cannot compensate for a cable that sticks. If the cable condition or derailleur alignment is uncertain, a bicycle workshop can diagnose the system and restore reliable shifting.