A couple of thoughts...
Depends a lot on what they are not doing.
I have never had an employee finish the "ring customer up" interaction. Primarily because they get involved in another conversation while waiting for the right situation. They won't auto end a good chat to go do this assigned work apparently.
Sometimes they just won't do what you tell them to do, or stop doing it at the first chance. This is a game mechanic built in by EA I believe to make managing the store employees as part of the process. As Carl said in his retail guide, probably best to choose a high work ethic as the primary hiring trait.
If they "finish" a task they have been given, they will not auto start doing that same task the next time the situation arises. Their task assignment defaults back to no assignment. An example, I set my one worker to answer customer questions, but as soon as my two active sims move into those conversations to complete the sale, if there aren't any new customers that join in the conversation before the employee ends it, I need to assign him the task again of answering customer questions. Same thing with restocking. If they finish before something new is sold, they will need to be assigned it all over again.