Trying putting a shower into one of the regular public restroom areas (closed off by its own door) instead of your Sim's back room, so customers won't want to follow your Sim in there. Have you tried putting free baked samples out on the tables? That seems to keep the customers occupied in my shop. They should be busy either browsing or chatting with each other. Be sure that when your Sim goes to the back room, that you cancel any chat bubbles in the queue they have going with others. And be sure at least one Sim stays out front to greet and talk to customers. Hope that will help! :)
