My Dr and my CD have lost their stats they now are 1 star which is white and talent etc are 2 at best they were 15 ish and 5star, now going to buy new one's.
That would just be the star rating has dropped because you yourself have leveled up. It's the games way of telling you to hire better staff. 5 stars is the best of the best but they are hard to come by.
As James said, the star rating is just an indicator of the staff level relative to your level. Their actual skill level has not decreased, your manager level has increased, so now they are not the best you can hire anymore.
CEO & CTO