Hard to change down from 3rd to 2nd gear in a 2002 Holden Barina (same as Corsa)

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We have a 2002 Holden Barina (same as the Opel/Vauxhall Corsa). The car has a 5 speed manual transmission. Recently we have noticed that it is becoming increasingly difficult to change down from 3rd gear to 2nd gear. It now takes a lot of force to achieve this gear shift although there is no grinding or crunching. When the motor is not running, there is no resistance to changing down from 3rd gear to 2nd gear.