As an office manager, how do I get an employee to change the way they do their job?

medical opinion about slow,inattentive students,unproductive workers,as to what hinders them is a no mans land,her choices might be to seek medical help with the belief that she has to improve,,,and may be hamstrung by docs who do not recognize that simple foods that everyone else eats,are an occupational lead weight,it may be she has a physical problem incorrectly diagnosed by her primary care doc,or she drinks ,smokes dope, abuses meds prescribed.She may go home to care for a sick relative,showing up tired and wacked out for 9 to 5 routine,there is a lot of stuff that can be tried before deciding she's dead wood,Usually,if workers have the human spark that impels them to go out every day and do something,leadership can help this person produce more,if the spark ain't there,well,cut her loose