I cannot remove stopper & rocker-arm assembly completely from bathtub drain
An accumulation of hard water deposits and anything else that washed through the drain is probably binding it's removal. Pour some CLR or Lime-away into the overflow opening. After awhile chase it with hot water. It could also be that any articulating pieces aren't lining up to allow the arms to be removed. If the sections are hinged they need to pivot at a 90 degree angle to the tub face.