Don't know whether yours is cable or hydraulic but in either case I would suspect the operating mechanism first, I'm sure someone who knows will be along shortly.
corsa is cable operated and the adjustment is done on the end of the cable where it clips into the fork arm on top of the gearbox.
check the locking clip they tend to fall off on corsas and the cable looses adjustment.
you can adust the cable and add another nut to lock it if the locking clip is missing.