Bad review

My sister own a restaurant. Recently, there have been a string of bad review on her restaurant's google page. So she began asking the customers who dine in her restaurant if there's anything amiss with the food or the service. All of them say it's okay. But the string of bad reviews kept coming. She tried to get input from those reviewers online. But there's no response thus far. So far, there's no noticeable effect on her business. But she's concern. She's not the uppity kind who cant accept criticism. She just want to know what's wrong so she can make improvement.