marc , tiz alan from the uk , the first thing i would check is the detent plunger
remove the outer cover and you will see it at about 10 past , this has a lock nut , hichcocks keep them part no 111094 @ £4.50 ,
slacken the lock nut , unscrew the plunger and fit the new one , winding fully in then back 1/2 to 1 1/2 turns , the flat on the plunger has to sit flat , this corisponds with the screwdriver flat ,
work in 1/2 turn incriments
alan