Maybe the screw is backing out? They have been know to wear rapidly too.

You should only use the cable outer for adjusting free play in the cable. Loosen the bigger "nut" or thumbscrew, then screw in (tighten) the thinner hex shaped part (cable adjuster) the cable rests on until there is a minute (1/32th inch) amount of play between the cable outer and the cable adjuster, tighten the thumbscrew.

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