the star rating is based on players you already have at the club. So for example, if you only had Adebayor and Defoe as your strikers, and you had a 5 star potential striker, but then you bought Cavani, Falcao, someone who is better than Adebayor or Defoe, then the 5 star potential striker would drop, as it is now based on the fact you have the new, better striker!
Basically, the star rating is not based on how high/good the players potential is, as most people think, but how high it is in comparison to your current players! (That's why when you are managing in the lower league you get alot of 4/5 star rated players who are not actually that good!